Geology and Mineral Deposits of the 1.1Ga Midcontinent Rift in the Lake Superior Region – An Overview

Jim Miller Precambrian Research Center and Department of Geological Sciences

University of Minnesota Duluth, Duluth, MN 55812

Suzanne Nicholson

United States Geological Survey, Reston, VA 20192

Introduction For over a century, many geologic, geochemical, geochronologic, and geophysical studies have been

focused on the Mesoproterozoic (1.1 Ga) volcanic, intrusive, and sedimentary rocks that compose the Midcontinent Rift (MCR) in the Lake Superior region. In the past quarter century, in particular, the empirical data collected from such studies have vastly improved our understanding of the three-dimensional structure and tectono-magmatic evolution of the MCR. These studies have shown the Midcontinent Rift to be one the best preserved large igneous provinces of Precambrian age. Moreover, research has suggested that the MCR was influenced, if not initiated, by a starting mantle plume.

The geologic understanding of the MCR, which is well exposed in the Lake Superior region, is continuing to improve due to a steady commitment to bedrock geologic mapping principally conducted by the Minnesota Geological Survey (e.g., Jirsa et al., 2011), the Ontario Geological Survey (e.g., Hart and MacDonald, 2007), and the U.S. Geological Survey (e.g., Nicholson et al., 2004). These three entities are currently collaborating on compiling a digital geologic map of the MCR that will include related tables of geochemical and geochronologic data and mineral deposit information (Nicholson et al., in preparation).

An understanding of the deeper structure of the rift and interpretations of its tectonomagmatic evolution have benefited from numerous interpretive studies of geochemical, geochronologic and geophysical data collected on the rift. These data sets and some of the more notable studies include:

1) High-resolution aeromagnetic data and a dense array of gravity measurements (Chandler et al., 1989; Chandler, 1990; Thomas and Teskey, 1994; Allen et al., 1997; Daniels and Snyder, 2002; Chandler and Lively, 2007);

2) Deep-crustal seismic reflection profiles (Behrendt et al., 1988; Chandler et al., 1989; Cannon et al., 1989; Hinze et al., 1990; McGinnis and Mudrey, 1991; Hinze et al., 1992; also see papers in 1994 Special Issue of Canadian Journal of Science v. 21, no. 4);

3) Geochemical and radioisotopic analyses of MCR igneous rocks (BVSP, 1981; Brannon, 1984; Green, 1986; Green et al., 1987; Sutcliffe, 1987; Paces and Bell, 1989; Nicholson and Shirey, 1990; Miller and Weiblen, 1990; Klewin and Berg, 1990, 1991; Lightfoot et al., 1991; Shirey et al., 1994; Miller and Ripley, 1996; Shirey, 1997; Miller and Chandler, 1997; Nicholson et al., 1997; Wirth et al., 1997; Vervoort and Green, 1997; Vervoort et al., 2007; Hollings et al., 2007a, b & c; Rogala et al., 2007; Hollings et al., 2010, 2012);

4) High precision U-Pb dates of volcanic and intrusive rocks (Davis and Sutcliffe, 1985; Palmer and Davis, 1987; Davis and Paces, 1990; Heaman and Machado, 1992; Paces and Miller, 1993; Davis and Green, 1997; Zartman et al., 1997; Vervoort et al., 2007; Heaman et al., 2007; Hoaglund, 2010); and

5) Detailed paleomagnetic measurements (Roberston and Fahrig, 1972; Halls and Pesonen, 1982; Swanson-Hysell et al., 2009, 2011).

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The MCR hosts several types of world-class ore deposits that have been exploited for many centuries. It is well established that nomadic Paleo-Indians mined native copper deposits on the Keweenaw Peninsula and Isle Royale as far back as 5,000 years ago (Martin, 1995). The first mineral rush in the United States was triggered by an 1841 report of mineable quantities of native copper in MCR basalt flows on the Keweenaw Peninsula by Michigan State Geologist Douglas Houghton. Although native copper (and minor silver) mining ended in 1967 and related copper sulfide mining at White Pine near Ontonagon, Michigan ended in 1996, increased global demand for base and precious metals over the past decade has spurred a historic surge in the exploration of various ore deposit types associated with the MCR, with several properties set to begin mining soon.

Chief among the deposit types being considered for mining is the low grade Cu-Ni-PGE sulfide ore occurring along the base of the Duluth Complex in northeastern Minnesota. Intermittent exploration of these deposits since their discovery in the early 1950’s has proved out the largest undeveloped resource of copper on Earth (Eckstrand and Hulbert, 2007). Stepped up exploration activity over the past decade has brought several projects to the pre-feasiblity and environmental permitting stages with mining anticipated to commence over the next several years. Similar Cu-Ni-PGE deposits associated with the Coldwell Complex near Marathon, Ontario are also in the pre-feasibilty stage of development.

In 2002, the discovery of massive Ni-Cu-PGE sulfide in the Eagle deposit near Marquette, Michigan stimulated a new round of exploration activity focused on the search for small MCR-related ultramafic intrusions emplaced into sulfidic sedimentary rocks. Currently, this search has identified over a half-dozen related Ni-Cu-PGE prospects in the Lake Superior region, with the Eagle deposit already permitted to begin mining in 2013. The recent discovery of numerous ultramafic intrusions associated with the early stages of the MCR has prompted active petrologic research on these bodies (Heggie, 2005; Laarman,2007; Hollings et al, 2007b; Ding et al., 2010; Goldner, 2011; Foley, 2011;) and a reevaluation of the tectono-magmatic evolution of the rift (Heaman et al., 2007; Hollings et al., in press)

In the first part of this overview, we review the geology, structure, and igneous chemostratigraphy of the MCR, and present some current ideas about its tectono-magmatic evolution. In the second part, we summarize the salient characteristics of the various ore deposits types associated with the MCR in the Lake Superior region.

Geologic Setting of the Midcontinent Rift Although buried by younger Phanerozoic sediments over most of its 2500 kilometer length, the

arcuate, segmented path of the MCR is easily traceable along gravity and magnetic anomalies that project in two arms, southwest and southeast, of exposures in the Lake Superior region (Fig. 1). The intense gravity and aeromagnetic anomaly formed by the MCR is one of the most distinctive geophysical features of the North American continent (King and Zietz, 1971; Hinze et al., 1992). Bouger gravity anomalies associated with the MCR range from a positive anomaly of over 60 mgals over the eastern part of the Duluth Complex to less than -90 mgals on the eastern flank of the rift in western Wisconsin (Allen et al., 1997).

Along the southwestern arm, the rift crosses geologic provinces ranging in age from 2.8 Ga to 1.7 Ga (Van Schmus, 1992; Holm et al., 2007; Fig. 2). In the Lake Superior region, the rift cuts across Late Archean (2.8-2.6 Ga) granite-greenstone terranes of the southern Superior province. From Lake Superior south into Iowa the rift crosses several terranes within the Paleoproterozoic (1.85 Ga) Penokean Orogen. The northern part of the orogen is composed of sedimentary rocks deposited at the continental margin of the Superior craton: these sedimentary rocks display increased deformation and metamorphism to the south (Cratonic Margin Domain, Fig. 2). South of the Niagara fault zone, a major suture zone, the orogen

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consists of an island arc assemblage of volcanic, intrusive, and immature sedimentary rocks (Pembine-Wausau Terrane, Fig. 2), as well as a block of isotopically reset Archean rocks, including gneiss dated at 3.6 Ga (Marshfield Terrane, Fig. 2). South of another major suture zone, the Spirit Lake Tectonic Zone (Fig. 2), which is largely inferred from geophysical and isotopic data, the MCR cuts across the Yavapai Province, a 1.7 Ga accreted terrane composed dominantly of felsic igneous rocks. Metamorphism, deformation, plutonism, and gneissic doming associated with the Yavapai orogeny overprint much of the Penokean as far north as the southern shore of Lake Superior (Fig. 2).

Figure 1. Geophysical characteristics of the southwest arm of the Midcontinent Rift in the north-central

United States. A) Bouger gravity anomaly map (dotted lines indicate buried crustal blocks inferred from gravity data (Kucks, 1999); see Fig. 3). B) Shaded-relief map of the total magnetic intensity anomaly (Bankey et al., 2002). The eastern arm of the MCR is deeply buried beneath Paleozoic rocks of the Michigan basin and has

been encountered in only a few deep drill holes (Brown et al., 1982). The gravity and magnetic signature of the MCR ends abruptly where as it crosses the geophysical trace of the Grenville front in southeastern Michigan at nearly a right angle. The Grenville front is a tectonic boundary marking the northwestern limit of penetrative deformation and metamorphic effects produced by the Elzevirian (1240 to 1160 Ma) and Ottawan orogenies (1090 to 1025 Ma), which together created the Grenville province (Easton, 1992). These two orogenies bracket the period of igneous activity and rifting of the MCR (1115-1086 Ma; Heaman et al., 2007). Cannon (1994) has suggested that rifting of the Midcontinent occurred during a period of diminished compression within the Grenville province whereas late tectonic inversion of the rift resulted from renewed tectonism (compression) during the Ottawan orogenic phase.

Structure of the Midcontinent Rift Extensive bedrock mapping of the MCR in the Lake Superior basin has provided a robust picture of

the present-day exposure of rift-related rocks (Fig. 3). This shows the rift to be composed of three major lithologic components: 1) a thick edifice of subaerial lava flows, 2) local concentrations of plutonic to hypabyssal intrusive rocks, and 3) an upper sequence of sedimentary rocks (Bayfield and Oronto Groups). More localized units occurring within pre-rift basement rocks include generally rift-parallel dike swarms, small ultramafic intrusions, and small alkaline and carbonatite intrusions. Also shown on Figure 3 of major faults that commonly juxtapose volcanic and sedimentary rocks.

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Figure 2. Geotectonic map of the north-central United States showing the major Precambrian terranes

cut by the Midcontinent Rift (modified from Holm et al., 2007).

When the surface features are integrated with the wealth of regional geophysical data collected over the MCR, especially the seismic reflection profiles collected across Lake Superior in 1986 for the GLIMPCE project (Great Lake International Multidisciplinary Program on Crustal Evolution; Behrendt et al., 1988), the result reveals the full, three-dimensional structure of the MCR. Seismic profiles across Lake Superior combined with Bouger gravity data indicate that the deepest part of the MCR lies beneath western Lake Superior, where the rift fill is as much as 36 km thick and where volcanic rocks comprise about two-thirds of the total (Cannon et al., 1989; Trehu et al., 1991; Hinze et al., 1992; Thomas and Teskey, 1994; Allen et al., 1997; Fig. 4). A minimum estimate for the volume of mafic rock in the Lake Superior region is 1.3x106 km3 (Hutchinson et al, 1990), whereas Cannon (1992) suggest a more realistic estimate is over 2x106 km3. This volume is comparable to many post-Mesozoic, continental-based, large igneous provinces (LIPs ) (Table 1).

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These geophysical data also suggest that a volume of magma nearly equivalent to that filling the rift, underplated the crust such that complete crustal separation probably occurred at least in the western Lake Superior area (e.g., Fig. 4; Behrendt et al., 1988; Trehu et al., 1991; Hinze et al., 1992; Allen et al., 1997). This at least doubles the amount of magmatic fill, as observed in other LIPs such as the Karoo-Ferrar igneous province associated with the rifting of Antarctica from southern Africa (fill = 5x106 km3, fill + underplate = 10x106 km3, Ernst and Buchan, 2001). Considering the rift fill, the volume of underplated material, and the unknown amount of eroded material, the Midcontinent Rift clearly is a world-class large igneous province.

Removing the effects of the late compression, the extensional structures of the rift also reveal themselves to be complex. Geophysical models across the MCR in the western Lake Superior region and along its southwestern arm, suggest that it was composed of a series of en echelon asymmetric grabens that change their polarity across subtle to well-defined accommodation zones (Chandler et al., 1989; Cannon et al., 1989; Dickas and Mudrey, 1997; Anderson, 1997; Berendsen, 1997). The trans-rift Thiel fault in central Lake Superior (Fig. 3) is an example of such an accommodation zone. Most of the volcanic rocks of the rift are confined within large-scale reversed faults that have inverted the grabens into horst structures (e.g., Keweenaw, Douglas, and Isle Royale faults, Fig. 3). Whereas 10 to 20 kilometers of volcanic rocks commonly are contained within the graben/horst structures, lava accumulations outside the bounding faults rarely exceed 5 kilometers (e.g., Fig. 4). This indicates that the reverse faults must have originally acted as normal growth faults and perhaps magma conduits to rapidly subsided and in-filled axial grabens (Cannon, 1992). Moreover, whereas volcanic rocks outside the central graben are predominantly older lava accumulations of reversed paleomagnetic polarity, younger normal polarity lavas comprise most of the exposed volcanic sequences within the grabens in the western part of the MCR (Figs. 3 & 4). This implies that graben formation began some time after the initiation of volcanic activity (Cannon et al., 1989). Interestingly, beneath the eastern part of the Lake Superior basin, where the axial graben is not bounded by distinct normal faults as in the west, older reversed polarity lavas are interpreted to comprise most of the 15-kilometer-thick volcanic sequence (Mariano and Hinze, 1994).

In western Lake Superior, the rift structure is further complicated by the effects of large, crustal blocks isolated within the volcanic basins (Fig. 3). Integrated modeling of gravity, magnetic, and seismic data over western Lake Superior (Sexton and Henson, 1994; Allen et al., 1997) has identified two areas within the axial part of the rift where the volcanic section pinches out. These areas are presumed to be large blocks or ridges of granitic crust, called the Grand Marias block and White's ridge (Figs. 1A & 3). These blocks may represent detached pieces of crust that became isolated during crustal separation. During volcanism, they stood as structural highs and exerted significant control on the shape of the graben in which the lavas accumulated, particularly the Portage Lake Volcanics (PLV). Allen et al. (1997) demonstrated that the axis of the central rift basin is centered between the Keweenaw Peninsula and Isle Royale and then curves around the Grand Marais block to the northwest toward the Minnesota coast. Miller and Chandler (1997) have suggested that the western growth fault margin of the PLV-equivalent rocks in Minnesota corresponds to the Finland Tectonomagmatic Discontinuity (FTMD) - an extensive arcuate dike and sill complex that is part of the hypabyssal Beaver Bay Complex (Fig., 3). White's Ridge further divides the western Lake Superior from another deep trough of volcanics to the southwest (Fig. 3).

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Figure 4. Geologic crustal model across the central part of Lake Superior based on seismic reflection data

along GLIMPCE Line A (shown in Figure 3) and Bouger gravity data (after Trehu et al., 1991 and Thomas and Teskey, 1994). IR and KF denote the Isle Royale and Keweenaw Faults, respectively. High density lower crust is interpreted to be mafic underplated material.

The Duluth Complex and related intrusions in northeastern Minnesota (Fig. 3) also cause the MCR to deviate from a linear graben form. Modeling of Bouguer gravity data over the Duluth Complex, which is characterized by two broad highs of greater than 50 mgals (Fig. 1A), indicates that the complex extends to a depth of about 13 km (Allen et al., 1997). The saddle between the two gravity highs has been attributed to another granitic crustal ridge, the Schroeder-Forest Center Ridge (SFC, Fig. 3; Miller and Chandler, 1997; Peterson and Severson, 2002), which divides the complex into two intrusive "basins". Although centered off the main axis of the rift, this accumulated thickness of magma is more than half of that which ponded in the central rift graben. Modeling of gravity data at the northern apex of Lake Superior by Thomas and Teskey (1994) suggest that a large mafic igneous complex with a thickness of as much as 20 km lies buried beneath the base of the Osler Group (Fig. 4). The reason that such large volumes of mafic magma ponded along the northwestern margin of the MCR is unclear but may be related to structural features of the pre-Keweenawan basement (Fig. 2). The Duluth Complex lies near the projection of the Penokean tectonic front and the Great Lakes tectonic zone (an Archean suture zone), along the projection of major Archean fault zones such as the Vermilion fault, and at the northern shelf margin of the Early Proterozoic Animikie basin. More specifically, sheet-like intrusions of the Duluth Complex and related bodies in Ontario appear to have been emplaced along the nearly concordant interface of subhorizontal Early Proterozoic sedimentary rocks of the Animikie Group and lava flows within the Keweenawan Supergroup.

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Volcanic Sequences of the Midcontinent Rift Stratified volcanic and sedimentary rocks contained within the MCR are collectively known as the

Keweenawan Supergroup (Morey and Green, 1982). Although faulting and burial of thickened portions of the rift fill beneath Lake Superior preclude studies of a continuous and complete sequence of the rift stratigraphy, its main components may be pieced together from exposures around the Lake Superior basin (Fig. 3). Physical and lithologic attributes of the main volcanic and sedimentary sequences exposed around the Lake Superior basin are summarized in Table 1. A proposed chronostratigraphic correlation of the main volcanic sequences and bounding sedimentary units are shown in Figure 5. This compilation is a modest revision of the most recent correlation proposed by Hollings et al., 2007b, which is slightly modified from Nicholson et al. (1997).

Prior to 1985, the principal means of correlating MCR volcanic sequences in the Lake Superior region was by their magnetic polarity. In the western Lake Superior basin, most volcanic rocks were correlated relative to a single magnetic polarity reversal from early reversed to late normal (Figs. 3, 4 and 5). However, at Mamainse Point in eastern Lake Superior, an additional normal and reversed interval was noted (Annels, 1973; Klewin and Berg, 1990). In the past two decades, high-resolution U-Pb geochoronologic studies of zircons and badellyites have replaced magnetic polarity as the main correlation tool for the volcanic and intrusive rocks of the MCR and have proven to be very important in the rapid expansion of our current knowledge of the rift. The age of the major R-N reversal is now constrained to have occurred between 1105 and 1102 Ma based on U-Pb ages for the uppermost reversely polarized lava in the Osler Volcanic Group, the Agate Point rhyolite (1105.3±2.1Ma, Davis and Green, 1997) and the oldest normally polarized age for the granitic rocks of the Mellen Complex (1102±2 Ma, Zartman et al., 1997). Until very recently, no dateable material had been found in the Mamainse Point sequence, despite several attempts. Therefore, attempts to correlate its seemingly more complete volcanic package (based on extra polarity reversals) with western volcanic sequences had to rely on gross magnetic polarity and geochemical attributes (Nicholson et al., 1997; Shirey et al., 1994). However, an as yet unpublished, high precision U-Pb zircon age on a volcanic tuff at the top of the uppermost reversed polarity sequence, just below the Great Conglomerate, is reported to peg the top of the reversed sequence at around 1100 Ma (Hysell-Swanson, personal comm., May 2012).

One of the more distinctive attributes of the MCR compared to other mantle plume-influenced, large igneous provinces, is the prolonged period of magmatism - almost 30 million years. The current collection of over 80 high precision U-Pb ages indicate that magmatic activity that can be geochemically linked to the development of the MCR occurred between 1115 Ma and 1086 Ma (Fig. 5). Heaman et al. (2007) suggested that intrusions in northwestern Ontario emplaced between 1150 and 1130 Ma may also be related to the MCR, but adding these diverse compositions to the MCR magmatic episode more than doubles the duration of MCR magmatism and complicates the generally coherent picture of its geochemical evolution (Nicholson et al., 1997).

The MCR volcanic sequences are composed of predominantly subaerially-erupted, tholeiitic flood basalts, but also include intermediate and felsic flows and fluvial interflow sedimentary rocks (Green, 1982). The lithostratigraphy of the main volcanic sequences are schematically portrayed in Figure 5 with volcanic intervals being subdivided into dominantly primitive basalts (mg#s >50), dominantly evolved basalts to basaltic andesites (mg#s <50), mixed volcanics of diverse composition (basalt to rhyolite), and rhyolite flows. Although the lithostratigraphies of the main volcanic sequences are distinct from one another in detail, there is a surprising commonality in their general chemostratigraphy (Nicholson et al., 1997). Based on the correlations shown in Figure 5, generalized stratigraphic variations of select

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lithochemical and isotopic compositions of mafic to intermediate lavas through the various volcanic packages are summarized in Figure 6.

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Figure 5. Chronostratigraphic correlation of the main volcanic sequences and bounding sedimentary units of the MCR in the Lake Superior basin. Also shown are main polarity intervals and U-Pb ages for volcanic and intrusive rocks (ages from Davis and Sutcliffe, 1985; Palmer and Davis, 1987; Davis and Paces, 1990; Heaman and Machado, 1992; Paces and Miller, 1993; Davis and Green, 1997; Zartman et al., 1997; Smyk et al., 2006; Vervoort et al., 2007; Heaman et al., 2007; Hoaglund, 2010; Ding et al., 2010; Goldner, 2011; Hollings et al., 2010; and Swanson-Hysell, pers. comm., 2012). Ages of intrusion are plotted to the left of volcanic sequence or pre-rift terranes into which they are intruded. Labels for some intrusion ages are: MC-Mellen Complex, E-Eagle, BBC-Beaver Bay Complex, DC-Duluth Complex, T-Tamarack, H-Hele, CC-Coldwell Complex, D-Disreali, JIS-Jackfish Lake Sill, S-Seagull, K-Kitto. Simplified lithostratigraphies of the four main volcanic-sedimentary packages are schematically portrayed; for details see the references listed in Table 1. The bold dashed lines indicate where the sequence is truncated by intrusions or major faults. Question marks indicate that the upper or lower age limit of the unit is unknown. Unit abbreviations for the Upper Michigan/NW Wisconsin package are: BSs-Bessemer Quartzite, SC-Siemens Creek Volcanics, LKC-Lower Kallander Creek Volcanics, UKC- Upper Kallander Creek Volcanics, PLV-Portage Lake Volcanics, SCV-St. Croix Volcanic Group, PM- Porcupine Volcanics, CHCg-Copper Harbor Conglomerate, LST-Lake Shore traps, NSh-Nonesuch Shale, and FSs-Freda Sandstone. For the North Shore Volcanic Group (NSVG) in northeastern Minnesota, which Green (2002) subdivides into two lithologically distinct, structural limbs, the unit abbreviations are: NSs-Nopeming Sandstone, PSs-Puckwunge Sandstone, LSW-Lower southwest sequence, LNE-Lower northeast sequence, USW-Upper southwest sequence, and UNE-Upper northeast sequence. The UNE and USW sequences are unconformably capped by the Schroeder-Lutsen sequence (SL). For the northwestern shore of Ontario (Black Bay Peninsula), the unit abbreviations are: SIF-Simpson Island formation, OVL-Osler Volcanic Group-lower suite, OLC-Osler Volcanic Group-central suite, and OLU-Osler Volcanic Group-upper suite. Upsection of the Osler Volcanics, southeast-dipping basalt flows on Isle Royale can be lithologically (and seismically) correlated with the northwest-dipping Portage Lake Volcanics (PLV) and overlying Copper Harbor Conglomerate (CHCg) exposed on the Keweenaw Peninsula (Fig. 3) and thus are given the same unit names (Huber, 1973). For volcanics of the Mamainse Point Volcanic Group exposed along the northeast Ontario shoreline, Klewin and Berg (1990) subdivide the sequence into 8 numbered intervals based on their lithologic and geochemical attributes. Several thick interflow volcaniclastic conglomerate units within the Mamainse Point section include: BCCg-basalt clast conglomerate, GCg-great conglomerate, and DCCg-Deadman’s Cove conglomerate. The Michipicoten Island Formation described by Annels (1974) is identified as MI. Recognition that the frequency of eruption and the compositional characteristics of MCR

magmatism was not constant has led many workers to subdivide the magmatic activity of the MCR into several stages (Sutcliffe, 1987; Shirey et al., 1994; Miller and Vervoort, 1996; Davis and Green, 1997; Nicholson et al., 1997; Vervoort et al., 2007; Heaman et al., 2007). Five magmatic stages are proposed in Figures 5 and 6 based on current geochronology, lithologic and geochemical data. The main attributes of these stages are as follows:

Initiation Stage (1115-1110 Ma) – this stage is represented by ultramafic to mafic intrusions that occur dominantly in the Thunder Bay-Lake Nipigon area of Ontario (Heaman et al., 2007) and by a buried intrusion in Michigan (Echo Lake gabbro: Cannon and Nicholson, 2001). No volcanics older than 1108±2 Ma have been dated in the MCR, though ages from the lowest parts of the volcanic sequences have yet to be acquired. Geochemical characteristics of picritic lavas of the lower parts of the Mamainse Point, PowderMill, North Shore, and Osler volcanic sequence are similar to estimated parent magmas of some early MCR ultramafic intrusions (Goldner, 2011; Foley, 2011), which may imply their being comagmatic and possibly contemporaneous. Given the thick sequence of reversed polarity lavas geophysically modeled in the eastern half of the Lake Superior basin (Mariano and

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Hinze, 1994) and in deep keel of the western basin (e.g., Fig. 4; ), it seems possible that pre-1110 Ma volcanics may lie buried there.

Early Stage (1110-1106Ma) – this stage is represented by reversed polarity lavas and intrusions of diverse compositions (ultramafic to felsic). All reversed volcanic sequences show similar chemostratigraphic sequences where early primitive basalts (mg#>50) give way to more diverse compositions that show evidence of crustal contamination (εNd < -2, Th/Yb>1; Figs. 5, 6). Rhyolites with distinctly negative εNd values, thought to indicate crustal anatexis of Archean to Paleoproterozoic crust (Vervoort and Green, 1997; Vervoort et al., 2007), also begin to appear later in this early stage of volcanic activity. Nicholson et al. (1997) noted that geochemical characteristics of lower reversed sequences in western Lake Superior (NSVG, Powder Mill Group, and Osler Volcanic Group, Figs. 5, 6) can be reasonably correlated. However the basal western Lake Superior composition (Lower Siemens Creek) is not represented among the lower Mamainse Point units (MP1 and MP2, Fig. 5). Instead, the basal Mamainse Point basalts are geochemically similar to the Upper Siemens Creek in western Lake Superior.

Hiatus Stage (1105-1101Ma) – this stage is characterized by a cessation of mafic magmatism and only intermittent felsic magmatism. An interesting aspect of the felsic magmatism during this stage is that it is best represented by the abundant occurrence of felsic clasts in the Copper Harbor Conglomerate in Upper Michigan (Fig. 5; Davis and Paces, 1990). Detrital zircons analyzed from a basal sandstone unit in the Copper Harbor Conglomerate yield ages ranging from 1106-1101 Ma (Davis and Paces, 1990). The paucity of significant felsic units in the underlying Portage Lake Volcanics suggests that these clasts (and zircons) were shed from rhyolitic composite volcanoes that were evidently eroded and not preserved in the volcanic rock record. Miller and Vervoort (1996) initially termed this the Latent Magmatic Stage so as to reflect their interpretation that this stage represents a period of magmatic underplating of the crust that is implied by geophysical models (e.g., Figs. 4 & 7). Hiatus stage is preferred because it is more descriptive and non-genetic.

Main Stage (1101-1094 Ma) – the bulk of preserved volcanic and intrusive rocks filling the MCR were emplaced during this stage, which occurred during a period of normal polarity. Although the magmatism involved a variety of magma compositions from primitive basalts to rhyolites as in the early stage magmatism, the mafic and minor intermediate compositions of the main stage show little evidence of crustal contamination (εNd =+2 to -2; Th/Yb <1; Fig. 6). Nicholson et al. (1997) distinguished two basalt composition types within the PLV and normal polarity NSVG sequences based on mostly on TiO2 abundance and mg#, with less evolved, low TiO2 compositions being predominant. The uppermost parts of the NSVG and PLV sequences are commonly capped by primitive olivine tholeiitic basalts with very low incompatible element abundances. In the NSVG, this interval is represented by the Schroeder-Lutsen basalt sequence, which sits unconformably on the Upper Northeast (UNE) and Upper Southwest (USW) sequences. This composition also occurs in the upper part of the Mamainse Point section (MP7).

Late Stage (1094-1086 Ma) – this stage is characterized by intermittent and localized volcanic activity in a period otherwise dominated by deposition of immature detrital sediments. These eruptions are dominated by intermediate to felsic magmas. Although no upper crustal magmatism evidently occurred after 1086 Ma, migration of copper-bearing crustal fluids occurred along reversed faults as late as 1040 Ma (Bornhorst et al., 1988) and locally resulted in the deposition of native copper and silver within the upper reaches of the volcanic pile.

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Figure 6. Magmatic evolution of the Midcontinent Rift interpreted from geochronology,

lithostratigraphy (Fig. 5), and chemostratigraphy of volcanic sequences and intrusive rock suites (Fig. 3). Data includes that referenced in Figure 5. Attributes of the five magmatic stages are discussed in the text.

The significance of these magmatic stages in the context of the overall tectono-magmatic evolution of the MCR will be discussed in a later section (Fig. 8). A more complete description of evidence for the apparent hiatus in mafic magmatism is warranted here, however. Evidence for the Hiatus Stage is found among all exposed volcanic sequences in the western Lake Superior basin (Fig. 5). Age dating of

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volcanic sequences straddling the major magnetic reversal shows a dramatic diminishment of eruption rates and a dominance of felsic volcanism. In the Wisconsin-Michigan border area, the major magnetic reversal occurs within the upper part of the 2- to 4-km-thick Kallander Creek Volcanics, which forms the upper formation of the Powder Mill Group (Fig. 3) and are dominated by intermediate to felsic flows (Nicholson et al., 1997). Two rhyolite flows which are stratigraphically separated by about 2 km and straddle the major magnetic reversal, yield ages of 1107.3±1.6 Ma (Davis and Green, 1997) and 1098.8±1.9 Ma (Zartman et al., 1997). In Minnesota, Davis and Green (1997) found a similar age difference (1107.6±2.5 - 1100.5±1.9 Ma) over a 350 m interval of mostly rhyolite and trachybasalt situated midway in the 6+ km-thick northeast limb of the North Shore Volcanic Group (Green, 1972). Thus, over half of the 13 million years of geologic time that is represented by the North Shore Volcanic Group is contained within less than 6% of its total stratigraphic thickness. Direct evidence for a similar diminution in mafic volcanic activity is not available from the Osler Group in Ontario (Fig. 3). However, a U-Pb age of 1105.3±2.1 Ma (Davis and Green, 1997) for a thick rhyolite flow that defines the upper part of the magnetically reversed section and separates two geochemically distinct basalt flow sequences, the upper and central suites (OGU and OGC in Fig. 5; Lightfoot et al., 1991), is consistent with such a possibility. Although no U-Pb ages of any type are found in the range of 1105-1102, Davis and Paces (1990) reported ages between 1106-1101 Ma for detrital zircons from a sandstone lense at the base of the Copper Harbor Conglomerate directly overlying the Portage Lake Volcanics on the Keweenaw Peninsula (Davis and Paces, 1990). This implies that any volcanoes that existed during the Hiatus Stage were exclusively felsic in the western part of the MCR. As described in more detail in the next section, U-Pb ages of intrusive rocks of the MCR indicate a similar hiatus in mafic magmatic activity between 1105 and 1101 Ma (Fig. 5).

Whereas the reduction in magmatic activity between 1107 and 1102 Ma appears to be real for the volcanic and intrusive suites in the western part of the MCR, it is not clear whether this reduction in mafic magma eruption is representative of the MCR as a whole. The recognition of two extra paleomagnetic reversals in the Mamainse Point Formation (Annells, 1973) may indicate more or less continuous eruption in the eastern basin, during the period between 1107 and 1100 Ma, while the western basin was in a relatively dormant stage. Previous attempts to correlate the Maimanse section with the western Lake Superior sequences interpreted the deposition of the polymict Great Conglomerate as likely correlative with the volcanic hiatus evident in the western sequences (e.g., Miller et al., 1995; Nicholson et al., 1997; Hollings et al., 2010). However the recent age of about 1100 Ma reported from just below the Great Conglomerate (Hysell-Swanson, pers. comm.., 2012) indicates the unit was deposited at the onset of the main magmatic stage.

Integration of geochronologic and geophysical data has allowed for reasonable estimates of the rates of local volcanism and basin subsidence. Assuming the present volume of volcanics in the rift to be about 1.5 million km3 and an originally erupted volume of at least 2 million km3 (taking erosion into account), the average eruption rate for the MCR would have been about 0.15 km3/yr (Cannon, 1992). However, if an approximately 5 Ma hiatus in mafic volcanic activity is factored in, eruption rates during the early and main stages of volcanic activity were probably greater. For example, eruption rates of up to 0.2 km3/yr have been calculated for the thick Portage Lake Volcanics, assuming a total volume of about 500,000 km3 emplaced over a 2.2 m.y. period (Davis and Paces, 1990; Cannon, 1992). These rates are greater than modern ocean plume environments (Iceland 0.05 km3/yr; Hawaii 0.03-0.1 km3/yr), but are comparable to some continental flood basalt provinces (Columbia River 0.07-0.28 km3/yr; Parana, >0.24 km3/yr), and less than others (Deccan 0.4-1.0 km3/yr) (Swanson et al., 1975; Gallagher and Hawkesworth, 1994).

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Based on similar data, Davis and Paces (1990) calculated a subsidence rate of 1.3 mm/yr for a 2850 m-thick interval of the Portage Lake Volcanics. Cannon (1992) calculated a similar value of 1.5 mm/yr for the average subsidence rate during all of main stage volcanism and estimated a range from 0.5 to 5 mm/yr. He estimated that subsidence after main stage volcanism slowed to an average rate of about 0.2 mm/yr. Eruption and subsidence rates during reversed polarity early stage volcanism was probably even greater. Although reversed volcanic sequences do not exceed 5 kilometers of exposed stratigraphic thickness (1.5 km - Mamainse Pt, Units 1-4, Annells, 1973; 2.4 km - Osler Gp, Lower and Central Suites, Lightfoot et al., 1991; 2.6 km - NSVG, lower NE sequence, Green, 2002; 4.8 km Powder Mill Group, Nicholson et al., 1997), U-Pb ages from reversed lava sequences tightly cluster in the range of 1109-1107Ma. If the 5 km-thick Powder Mill Group erupted within 2 million years, this would imply a subsidence rate of 2.5 mm/yr. Despite only 1.5 kilometers of the 5 km-thick Mamainse Point section being composed of reversed volcanics, modeling of magnetic, seismic and gravity data in the eastern lake basin (Mariano and Hinze, 1994a and b) indicates that reversed lavas thicken dramatically toward the axis of the rift basin where they comprise a thickness of up to 10 kilometers. If this thickness was erupted over the same 2 Ma timeframe as reversed lavas in the western part of the rift, it would imply a rate of lava accumulation of 5 mm/year. However, the recent discovery of reversed polarity intrusive rocks as old as 1117 Ma in the Lake Nipigon area (Heaman et al., 2007) hints that similar-aged volcanic rocks may lie at the base of reversed volcanic sequence in the eastern basin. Moreover, the discovery that the top of the uppermost reversed sequence at Mamainse Point has an age of about 1100 Ma (Hysell-Swanson, pers. comm., 2012) increases the upper age limit of revered polarity lavas to well above 1106 Ma. Therefore, the inference of a 2 Ma eruption window for reversed polarity volcanism is unlikely.

Intrusive Rocks of the Midcontinent Rift Intrusive igneous rocks associated with the development of the MCR are considered part of the

Midcontinent Rift Intrusive Supersuite (Miller et al., 2002a,b). In his summary of MCR intrusions, Wieblen (1982) identified three general categories of intrusions that can be distinguished by volume, age range, emplacement history, geographic distribution, compositional range, and country rocks. These are 1) large subvolcanic intrusive complexes, 2) isolated alkalic and carbonatitic intrusions, 3) mafic dike and sills swarms. Since Weiblen’s compilation, a fourth type of intrusion has been identified as an important and distinctive class of intrusions - small ultramafic/mafic intrusions which commonly host Ni-Cu-PGE mineralization.

Subvolcanic Intrusive Complexes

The main subvolcanic intrusive complexes exposed in the Lake Superior basin are the Duluth Complex and Beaver Bay complexes emplaced into the North Shore Volcanic Group of northeastern Minnesota (Miller et al., 2002a,b) and the Mellen Complex emplaced into the Powder Mill Volcanic Group near the Michigan-Wisconsin border (Fitz, 2011; Fig. 3). All complexes were emplaced through multiple intrusive events of varied magma compositions into the lower to medial sections of MCR volcanic sequences. Although the difference in size between the Duluth and Mellen Complexes (Fig. 3) reflects, in part, the steeper rift-ward dip of the Mellen Complex, gravity data indicate that the Duluth Complex is significantly larger in volume. The Duluth Complex has a surface area of over 5,000 km2, is situated over two of the largest Bouger gravity anomalies (>50 mgals) associated with the MCR, and is estimated to be rooted to a depth of as much as 13 kilometers (Allen et al., 1997). This implies a total volume between 35,000 and 40,000 km3. Multiple mafic to felsic intrusions emplaced higher in the NSVG volcanic

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edifice in northeastern Minnesota are distinguished from the deeper-seated Duluth Complex intrusions and are termed the Beaver Bay Complex and miscellaneous subvolcanic intrusions (Fig. 3).

The Mellen Complex of Wisconsin (Fig. 3) is a 1- to 5-kilometer thick, sheet-like complex composed four main intrusions – the Potato River intrusion to the east, the Mellen granite in the center, Mineral Lake intrusion in the west, and the small Rearing Pond intrusion in the upper part of the Mineral Lake body. All were emplaced into the reversed polarity Powder Mill Group Volcanics during main stage (normal polarity) magmatism, though the 4.5-kilometer-thick Mineral Lake intrusion is floored by Paleoproterozoic and Archean rocks. Both the Mineral Lake and Potato River intrusions include lower olivine gabbroic cumulates, a thick midsection of gabbroic anorthosite cumulates, and a cap of granophyre. Granophyre from the Mineral Lake Intrusion yields a U-Pb age of 1102±2.8 Ma (Zartman et al., 1997). The Rearing Pond intrusion is a well differentiated layered mafic intrusion that grades from a lower dunite to an upper ferrogabbro (Olmsted, 1969). The Mellen granite is unlike any other felsic pluton intrusion in the MCR by virtue of its intergranular texture and preponderance of biotite; MCR granophyres typically display micrographic texture and contain prismatic ferropyroxene as the dominant mafic phase. The Mellen granite has a U-Pb age of 1100.9±1.4 Ma (Zartman et al., 1997) which, by the fact that it contains gabbro inclusions evidently derived from adjacent mafic intrusions, places an upper limit on the age of the Mellen Complex.

The Duluth Complex is composed of multiple mafic and felsic, sheet-like intrusions that were emplaced into the base of the NSVG volcanic edifice during the early and main magmatic stages of the MCR (Miller and Weiblen, 1990; Miller and Ripley, 1996; Miller and Severson, 2002b). Duluth Complex intrusions are routinely classified into four series based on their bulk composition, age, and internal structure – felsic, early gabbro, anorthositic, and layered (Fig. 7). The earliest intrusions include granophyric granite bodies of the felsic series, which are concentrated along the complex’s upper contact, and gabbroic to ferrodioritic mafic layered intrusions of the early gabbro series in the eastern arm of the complex. Both series display reversed polarity and yield U-Pb ages between 1109 and 1106 Ma (Paces and Miller, 1993; Vervoort et al., 2007). Where in contact, field relationships consistently imply that emplacement of granophyre preceded the gabbroic intrusions. The bulk of the exposed area of the Duluth Complex (>75%) was formed during the onset of the main stage of MCR magmatism with the formation of complex gabbroic to anorthositic cumulates of the anorthositic series and the emplacement of discrete, variably differentiated mafic layered intrusions of the layered series. Although field relationships consistently show that the anorthositic series was emplaced before the layered series, high precision U-Pb ages (errors < 0.5 Ma) of five layered series samples and four anorthositic series samples indicate that both series formed within a one million year period between 1099-1098 Ma (Paces and Miller, 1993; Davis and Green, 1997; Hoaglund, 2010). Hoaglund (2010) calculated the volume of layered series intrusions to conservatively be 15,000 km3 and the anorthositic series to be about 10,000 km3, which yields an average combined emplacement rate of about 0.025 km3/yr. Not knowing the amount of eroded material, this may rate may be considerably greater (double?).

The Beaver Bay Complex (BBC) is a hypabyssal, multiple-intrusive igneous complex exposed over a 600 km2 area in northeastern Minnesota (Fig. 7; Miller and Chandler, 1997; Miller and Green, 2002a). The BBC and related hypabyssal intrusions were emplaced into the medial section of the North Shore Volcanic Group (USW and UNE sequences in Fig. 5). Thirteen intrusive units have been identified within the BBC, representing a minimum of six major intrusive events. With the exception of a body of granophyre, most intrusions were formed from gabbroic to dioritic parental magmas with successive intrusions generally involving less evolved compositions. U-Pb ages (Paces and Miller, 1993; Hoaglund, 2010) indicate that most BBC intrusions were emplaced at about 1096 Ma, about 3 million years after the

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Duluth Complex (Fig. 5). However, the earliest intrusions of the BBC, as implied from field relations, have not yet been dated.

Figure 7. Geology of northeastern Minnesota showing the principal MCR-related intrusive and volcanic

units. Also shown are the principal areas of Cu-Ni-PGE sulfide deposits. Alkalic and Carbonatitic Intrusions

MCR-related alkaline and carbonatitic rocks occur in several intrusive complexes emplaced into Archean rocks north of Lake Superior (Fig. 3). The largest of these are the Coldwell and Killala Lake alkaline complexes and the Prairie Lake carbonatite, but also includes numerous small lamprophyric, carbonatitic and alkaline intrusions (Sage, 1991). As described below, alkalic and carbonatitic intrusive rocks related to the MCR host base, precious and rare metal occurrences (Smyk and Sage, 1995).

Most of these complexes are spatially localized and structurally controlled by the Trans-Superior Tectonic Zone (TSTZ, Fig. 3) and the Kapuskasing Structural Zone (Fig. 3). The Trans-Superior Tectonic Zone is a north-northeast-trending structure that extends for over 600 km and appears to link up with the Thiel Fault in Lake Superior (Klasner et al., 1982). Dickas and Mudrey (1997) interpret the Thiel-TSTZ structure to be a major accommodation zone in the segmentation of the MCR. The Kapuskasing Structural Zone varies from a broad zone of faulting at the east coast of Lake Superior to a

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very narrow zone near Hudson Bay to the north. The zone contains west-dipping faults exposing an approximately 20 km thick section of Archean crust related to transpressive tectonics between 2.6 and 2.45 Ga (Percival and West, 1994). Sage and Watkinson (1995) suggested that these large-scale structures served to not only focus intrusion of alkalic and carbonatite magmas, but also were reactivated and played major roles in the structural development of the MCR.

Precise U-Pb ages for alkali and carbonatitic intrusions associated with the TSTZ range from 1150 to 1100 Ma (Smyk, 2010). The age dates plotted in Figure 5 are for the Coldwell Complex (1108 ± 1Ma, Heaman and Machado, 1993) and two carbonatite intrusions dated by Heaman et al. (2007) - Nemegosenda (1105.4 ± 2.6 Ma) and Lackner Lake (1100.6 ± 1.5 Ma.). Sage and Watkinson (1995) do not report precise ages for intrusions into the KSZ, but nevertheless interpret many of the small carbonatites, lamprophyres and alkaline intrusions to be related to the MCR.

The Coldwell Complex (Fig. 3) is the largest and most complex of the alkaline intrusions associated with the MCR. The sub-circular complex has a diameter of 25 km and covers an area of approximately 580 km2 (Good et al., 2010). The complex is composed of three, superimposed ring complexes or magmatic centers that become progressively more alkalic (Mitchell and Platt, 1978).

Center 1: gabbro and iron-rich augite syenite Center 2: alkalic biotite gabbro and nepheline syenite Center 3: syenite and quartz syenite

Center 1 gabbroic rocks forming the eastern part of the complex contain Cu-Ni-PGE mineralization that is currently being evaluated for development (see Good, this volume). The superimposition of intrusive centres and a complex and protracted magmatic history have produced a myriad of hybrid rocks, igneous breccias and ambiguous cross-cutting relationships. Hornfels volcanic inclusions occur throughout the complex (Smyk, 2010), implying that the complex was likely emplaced beneath an edifice of MCR volcanics.

Mafic Dike and Sill Swarms

Mafic dikes, which likely served as feeders to flood basalts and subvolcanic intrusions, are found within and peripheral to the MCR in the Lake Superior basin (Fig. 3). In addition, thick mafic sills, subconformable sheets, and small intrusions are common in shallow-dipping Paleoproterozoic to Early Mesoproterozoic sedimentary sequences in the Lake Nipigon area, the MN-ON border area, and eastern Mesabi Range.

Green et al. (1987) compiled the lithologic, geochemical, structural and paleomagnetic attributes of mafic dike swarms associated with the MCR (Fig. 3). Whereas mafic dikes are locally well exposed, especially in Ontario, dike swarms that occur in pre-rift basement rocks are easily recognizable from high resolution aeromagnetic images where they stand out as positive or negative linear anomalies. Dikes also cut MCR volcanic and intrusive rocks, but are not as easily recognized on magnetic anomaly maps due to more subdued contrast with the country rock. Dike swarms generally strike parallel to the trend of the rift and span mafic compositional ranges typically found in the basaltic flows of the MCR. The width of most dikes ranges from 10 - 50 meters, but some can be as wide as 500 m. Although scant age dates have been obtained from the dike swarms, both normal and reversed polarity dikes are evident in most swarms. Typically, the orientations of each polarity type in a given swarm are only slightly different.

In the MN-ON border area, Hollings et al. (2010, 2012) recently delineated several dike swarms with distinct orientations, magnetic polarities, and compositional attributes. The dominant swarm is the Pigeon River dike swarm, which trends east-northeast to northeast (rift parallel) and dips steeply to the southeast. Although dike compositions range in mg#, Hollings et al. (2010) distinguished two

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compositional types, a low TiO2-Gd/Yb group and, a less common, high TiO2-Gd/Yb group, which are similar to the compositional differences between nearby Logan and Nipigon Sills (see below). The Pigeon River dikes have not been dated radiometrically: however, they show normal magnetic polarity and thus are thought to have been emplaced during main stage magmatism. The northwest-trending Cloud River dike swarm is compositionally similar to the low TiO2-Gd/Yb Pigeon River dikes, but appears to be older as this swarm is reversely polarized and yields an U-Pb age of 1109.2±4.2Ma. The strike orientation of the large Pine River-Mt. Mollie composite dike curves from NE (where it is subparallel to the Pigeon River dikes) to E-W (where it appears to merge with the 1099.6±1.2Ma Crystal Lake gabbro). It has reversed polarity and yields an age of 1109.3±6.3Ma, similar to the Cloud River dikes.

Mafic sills are also common in the MN-ON border area and around Lake Nipigon where they produce a dramatic mesa-like topography. Although all MCR-related sills in this region were originally called Logan Sills (Sutcliffe, 1987; Smith and Sutcliffe, 1989), Hollings et al. (2007b, 2010) have shown that the sills in the Lake Nipigon area have more diverse compositions and are characterized by low TiO2 and Gd/Yb, whereas sills south of Thunder Bay are more consistently evolved (mg#<40) and have elevated TiO2 and Gd/Yb. They recommend distinguishing Nipigon Sills as those occurring north of Thunder Bay from the Logan sills to the south. Hart and MacDonald (2007) report four main Nipigon sills, ranging from <5m to >180m in thickness, occur as sub-conformable bodies intruded into the 13 Geon Sibley Group sedimentary rocks. Weiblen et al. (1972) and Smith and Sutcliffe (1989) report six major Logan Sills in the international border area where they occur as semi-conformable sheets within the sub-horizontal Paleoproterozoic (1.84-1.78 Ga) Rove Formation (Hollings et al., 2010). They typically range in thickness between 3m and 20m, with a maximum of 50 m.

An increasing number of small ultramafic to mafic intrusions related to the MCR have been discovered over the past decade due to expanded exploration for Ni-Cu-PGE deposits commonly associated with such intrusions. The currently identified intrusions (Fig. 3) typically occur in Paleoproterozoic metasedimentary rocks in Minnesota (Tamarack - Goldner, 2011) and Upper Michigan (Eagle - Ding et al., 2011; BIC - Rossell, 2008; Foley, 2011; Roland Lake – Schulz and Nicholson, 2009) or in Archean to middle Mesoproterozoic rocks in the Thunder Bay – Lake Nipigon areas of Ontario (Current Lake, Seagull, Disreali, Hele, Kitto, Shillabeer, Jackfish Island, and Riverdale - Hart and MacDonald, 2007; Goodgame et al., 2010; Hollings et al., 2007b, 2007c, 2010, 2012). Those that have been dated radiometrically consistently show these intrusions to be correlative with the initiation and early stage of MCR magmatism (1115 – 1105 Ma, Fig. 3).

Ultramafic-Mafic Intrusions

The size and shapes of the MCR ultramafic-mafic intrusions are quite variable. All intrusions are less than one kilometer thick and cover a limited area between 84 km2 (Seagull - Hollings et al., 2007) and 0.1 km2 (Eagle - Ding et al., 2010). Intrusion shapes include sheet-like (Shillabeer, Jackfish, Disreali, Hele, and Kitto), lopolithic (Seagull), bowl-shaped (BIC), elliptical cone-shaped (Eagle), and chonolithic (Current Lake, Tamarack). Although the unexposed, tube-like Current Lake intrusion north of Thunder Bay is known to be 50m to 600m wide and 6km long, a network of linear aeromagnetic anomalies related to it covers a 12km by 6km area that continues to be explored (Goodgame et al., 2010).

All intrusions contain a lower ultramafic section composed olivine and pyroxene cumulates that, in most cases, is overlain by mafic cumulates composed of plagioclase, pyroxene and commonly Fe-Ti oxide. The exceptions to this are the Eagle and Current Lake intrusions, which are entirely composed of

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lherzolitic (Ol+Opx+Cpx) cumulates with 15-40% intercumulus plagioclase (Ding et al., 2010; Goodgame et al., 2010). Where a mafic cap in the upper portion of these intrusions is missing, as at the Eagle intrusion, erosion of the upper part of intrusion may be the cause.. The lack of a mafic component to the Current Lake intrusion may be related to its narrrow tube-like shape, although, as found at Tamarack (Goldner, 2010), a more complete differentiated sequence may be found where the intrusion widens out (e.g., Southeast Anomaly zone (MacTavish and Smyk, 2010)). The specific cumulate paragenesis is not similar in all intrusions. For example, the Tamarack intrusion of Minnesota displays a cumulate paragenetic sequence of Ol Ol+Opx+Cpx Pl+Cpx+Opx Pl+Cpx+Opx+FeOx+Ol (Goldner, 2011). In contrast, the BIC intrusion of Upper Michigan shows a paragenetic sequence of Ol

Cpx+ Ol Cpx+FeOx Ol Pl+Cpx+FeOx (Foley, 2011). Another interesting feature of these intrusions is that most appear to have formed in two or more major emplacement pulses (Tamarack - Goldner, 2011; BIC - Foley, 2011; Eagle - Ding et al., 2010; Current Lake - Goodgame et al., 2010; Seagull - Heggie, 2005; Hele - Hollings et al., 2007a).

Because the ultramafic-mafic intrusions were emplaced into pre-MCR rocks, most intrusions develop marginal chill zones that, when corrected for accumulated olivine, can be used to estimate parent magma compositions (Goldner, 2011; Foley, 2011, Miller et al., 2011). These estimates indicate similarities in major and trace elements to picritic lavas commonly found in early reversed MCR lava sequences (Hollings et al., 2007b; Ding, 2010; Goldner, 2011; Foley, 2011). The volatile-rich compositions of these parent magmas are indicated by the abundance of primary amphibole, biotite, carbonate, and sulfide (Goldner, 2011; Foley, 2011; Heggie, 2005). Incompatible trace elements show a pronounced enrichment trend similar to OIB compositions, though moderately to slightly negative Nb-Ta anomalies are common (Hollings et al., 2007b; Goldner, 2011; Foley, 2011). Negative Nb-Ta anomalies are not uncommon to continental flood basalts in general (Campbell, 2001) and are common attributes of MCR volcanics (Nicholson et al., 1997). Such anomalies may indicate contamination of OIB-type (plume-generated) magmas with continental crust or by subcontinental lithospheric mantle possibly containing recycled crust from Archean subduction (Shirey, 1997; Hollings et al., 2007b, 2010, 2012).

Sedimentary Rocks of the Midcontinent Rift The largely fluvial redbed sedimentary rocks that fill the rift include four lithostratigraphic groups:

1) thin pre-volcanic, quartzose fluvial and lacustrine deposits; 2) syn-volcanic, interflow volcano-clastic sedimentary rocks; 3) post-volcanic, immature sedimentary rocks of the Oronto Group and equivalents; and 4) quartzose sandstone of the Bayfield Group and Jacobsville Sandstone (Ojakangas and Morey, 1982). The flat-lying Sibley Group sedimentary rocks exposed in the Lake Nipigon area and intruded by early MCR intrusions (Fig 3) are distinctly older than the MCR at 1350-1300 Ma (Franklin et al., 1980) and are unconformably overlain by pre-volcanic MCR sedimentary rocks (Hollings et al.,. 2007b). Although Franklin et al. (1980) speculated that the Nipigon Embayment represents a failed third arm aulocogen of the MCR, others (Fralick and Kissin, 1995; Hollings et al., 2004; Rogala et al., 2007) have suggested that the Sibley Group was deposited in a half-graben related to a 1350 Ma anorogenic thermal event, some 200Ma before the MCR.

Quartzose sandstone, siltstone, and minor quartz pebble conglomerate underlie early MCR volcanics in four different areas in the Lake Superior region (Fig. 5). These pre-volcanic units include: 1) the Bessemer Quartzite in the Wisconsin-Michigan border area occurring beneath the Powder Mill Volcanic Group, 2) the Nopeming Sandstone near Duluth, Minnesota occurring beneath the lower southwestern sequence of the North Shore Volcanic Group; 3) the Puckwunge Sandstone in near Grand Portage,

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Minnesota beneath the lower northeastern sequence of the North Shore Volcanic Group; and 4) the Simpson Island Formation forming the base of the Osler Volcanic Group northeast of Thunder Bay, Ontario. Bedding features of the Nopeming, Puckwunge, and Simpson Island units imply fluvial deposition in a braided stream environment, whereas bipolar paleocurrent indicators in the Bessemer Quartzite imply tidal or longshore influences (Ojakangas and Morey, 1982, Hollings et al., 2007b).

Coarse, immature, polymict sandstones and conglomerates (lithic arkose to feldspathic lithic arenite) occur as interflow sedimentary units throughout all volcanic sequences. Sources of detritus include felsic and mafic volcanic rocks, mafic intrusive rocks and non-MCR rocks outside the rift basin (Green, 2002). All paleocurrent indicators show generally basinward vectors (Merk and Jirsa, 1982). In the Mamainse Point Volcanic Group, interflow conglomerates and minor sandstones comprise almost one quarter of the volcanic-sedimentary sequence, with the Great Conglomerate unit itself being over 500m thick (Annels, 1973). Interflow sedimentary units in the Portage Lake Volcanic Group are well known because they host many of the principal native copper deposits on the Keweenaw Peninsula of Michigan. A total of 22 interflow units vary in thickness from tens of centimeters to as much a 120 meters and are dominated by conglomerates. These interflow units comprise about 3 percent of the total thickness of the Portage Lake Volcanics (Merk and Jirsa, 1982). On Isle Royale, interflow sedimentary units make up 10 to 15 percent of the Portage Lake sequence. Interflow sedimentary units in the North Shore Volcanic Group comprise about 4 percent of the sequence and tend to be dominated by sandstone (Jirsa, 1978; Green, 2002). The thickest unit is the 100m-thick Cut Face Creek sandstone that forms the base of the uppermost Schroeder-Lutsen Sequence and rests unconformably on older volcanics (Fig. 5).

During the waning of volcanic activity during the Late Magmatic Stage, continued subsidence of the rift grabens resulted in accumulations of up to 8 kilometers of conglomerate, sandstone and siltstone intercalated with localized volcanic eruptions (Cannon, 1992). These generally immature sedimentary rocks are collectively termed the Oronto Group. Where it is exposed in Michigan, Wisconsin and Isle Royale, the Oronto Group is subdivided into three formational units: the Copper Harbor Conglomerate, the Nonesuch Shale, and the Freda Sandstone. The Copper Harbor Conglomerate is a lens-shaped red bed unit with a maximum thickness of 1830 meters that fines upward and basinward from a volcanic clast-dominated conglomerate to lithic subarkose sandstone (Daniels, 1982). The unit is interpreted as a prograding alluvial fan complex developed basinward of graben-bounding faults (Keweenaw Fault and Isle Royale Fault in Fig. 3). The Nonesuch Shale, which interfingers with the upper Copper Harbor Conglomerate, is an unoxidized sequence of siltstone, shale, and sandstone with a high hydrocarbon and sulfur content (Daniels, 1982). The Nonesuch, which is up to 215 meters thick, is interpreted to have formed in a closed lacustrine basin. Its carbon- and sulfur-rich composition appears to have been key to the formation of stratiform Cu-sulfide deposits (Swenson et al., 2004), as will be discussed later. The Freda Sandstone is a red-bed sequence of arkosic to quartzose sandstone and shale up to 3660 meters thick (Daniels, 1982). It is interpreted to be fluvial in origin.

The final phase of sedimentation occurred when the extensional tectonics of rifting was replaced by compressional stresses generated during Grenville orogenesis between 1080-1040 Ma (Cannon, 1994). This northwest-directed compression caused graben-bounding normal faults to be transformed into reverse-thrust faults and resulted in an inversion of the rift with the creation of a central horst bounded by flanking basins. With the formation of a central horst in some segments of the rift basin, the Oronto Group sediments were locally reworked and deposited into the marginal basins along with sediments derived from outside the MCR basin. Geophysical models infer that sedimentary thicknesses in the marginal basins of as much as 3 kilometers (Hinze et al., 1982; Chandler et al., 1989; Allen et al., 1997). These feldspathic to quartzose red-bed sandstones are called the Bayfield Group in Minnesota and

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Wisconsin (Morey and Ojakangas, 1982) and the Jacobsville Sandstone in eastern Lake Superior (Kalliokoski, 1982). In northwestern Wisconsin, the Bayfield Group is subdivided into the feldspathic Orienta Sandstone, the quartzose Devils Island Sandstone, and the feldspathic Chequamegon Sandstone. In eastern Minnesota, the principal formational units are the feldspathic Fond du Lac Formation, and the more quartzose Hinkley Sandstone. All Bayfield Group/Jacobsville units are interpreted to have formed in fluvial to lacustrine sedimentary environments.

Tectono-magmatic Evolution of the Midcontinent Rift Although considerable debate has arisen over the past decade regarding the role, extent, and even the existence of mantle plumes (Buchan and Ernst, 2001; Foulger et al., 2005; Foulger and Jurdy, 2007), most workers on the Midcontinent Rift attribute its tectonic and magmatic evolution to the influence of a starting mantle plume (Hutchinson et al., 1990; Nicholson and Shirey, 1990; Cannon and Hinze, 1992; Shirey et al., 1994; Miller et al., 1995; Miller and Vervoort, 1996; White, 1997; Hinze et al., 1997; Davis and Green, 1997; Shirey, 1997; Nicholson et al., 1997; Wirth et al., 1997; Vervoort and Green, 1997; Hollings et al., 2007b; Heaman et al., 2007; Vervoort et al., 2007; Hollings et al., 2010; Hollings et al, 2012).

Most of these studies argue that the geologic, geochemical, geophysical and geochronologic attributes of Midcontinent Rift in the Lake Superior area are best explained by chemical and physical processes attending the arrival of an anomalously hot starting mantle plume at the base of the lithosphere at about 1115 Ma. Evidence in support of a plume model includes the estimated volume of more than 2 million km3of erupted material (Cannon, 1992), based on seismic reflection data, and a nearly equivalent amount of magma underplated and intruded into the crust as suggested by gravity data (Behrendt et al., 1990, Trehu et al., 1991; Hinze et al, 1992; Mariano and Hinze, 1994b; Thomas and Teskey, 1994). This volume of magma requires an anomalously hot mantle source such as would be expected from a mantle plume (Hutchinson et al., 1990). Cannon and Hinze (1992) argued that if the Nipigon Embayment is taken to represent the failed third arm of the MCR, as suggested by some (Franklin et al., 1980; Sutcliffe, 1987; Lightfoot et al., 1991), but questioned by others (Fralick and Kissin, 1995; Hollings et al., 2004; Rogala et al., 2007; Hart and MacDonald, 2007), a radial pattern of dike swarms that is centered near Thunder Bay would be consistent with a triple junction generated by the impact of a mantle plume. Radiogenic isotope and trace elements of most mafic rocks of the MCR are also consistent with their derivation from an undepleted mantle plume (Nicholson and Shirey, 1990; Shirey et al., 1994; Nicholson et al., 1997; Shirey, 1997; Hollings et al., 2007c; Hollings et al., 2010; Hollings et al., 2011). Indeed, the fact that the earliest volcanic products show undepleted mantle isotopic signatures at a time of minimal lithospheric extension demonstrates that a rising plume was involved in the rifting process from the onset and was probably an integral force that drove extension and thinning of the lithosphere (Cannon and Hinze, 1992; Nicholson et al., 1997).

The 30 Ma duration of MCR magmatism is perhaps its most difficult characteristic to rectify with a mantle plume model (Hill, 1991; Campbell, 2001; Ernst et al., 2005). The duration of plume-related magmatism associated with most Mesozoic to Tetiary LIPs is only a few million years (Coffin and Edholm, 1994; Ernst and Buchan, 2001b). This becomes an even greater problem if MCR magmatism is extended back to 1150 Ma as suggested by Heaman et al. (2007). Hollings et al. (2011) has suggested that this prolonged magmatism may indicate the MCR was affected by a mantle plume cluster as proposed by Ernst and Buchan (2002) to explain the 2.4 Ga long-lived Matachewan and Mistassini event. Another explanation for prolonged multimodal magmatism is that proposed by Bercovici and Mahoney (1994), whereby a starting plume head stalls at the 670 km discontinuity and creates a double-headed

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plume that subsequently results in two main phases of magmatism separated by tens of millions of years. The problem with any multi-plume model for the MCR is that paleomagnetic data imply that Laurentia was drifting at a rate of 20-40 cm/yr (Swanson-Hysell et al., 2009; this volume) making it unlikely in the case of a double-headed plume, and fortuitous in the case of plume cluster, that plume heads would impact the same spot on the lithosphere.

As pointed out by Campbell (2001), “the time difference between the onset of volcanism and the start of runaway extension is dependent on the strength of the lithosphere prior to the arrival of the plume and on the magnitude of the subduction-related forces acting on the plate. The observed difference can range from 1 to 20 m.y. (Hill, 1991)”. By the end of main stage magmatism (1094Ma), the plume component (e.g., basalts with εNd ≈ 0) appears to be waning and thus the period of “plume” magmatism may be closer to 20 Ma, a less problematic duration. A six-stage tectono-magmatic model is proposed here (Fig. 8) that attempts to account for the various geochemical, geochronologic, and structural attributes of the MCR by a single starting mantle plume impacting and embedding its plume head into the base of the Laurentian lithosphere at around 1115Ma.

Stage I (1115-1110 Ma) – Plume Impact and Crustal Doming - The recent discovery of a mix of mafic to ultramafic intrusions in the Nipigon Embayment area with emplacement ages between 1115 and 1110 Ma (Heaman et al., 2007) and an apparent lack of volcanic rocks of this age has compelled a rethinking about the timing and the tectonic effects of mantle plume impact with the base of the lithosphere. The lack of volcanic ages older than 1109 Ma (Fig. 5) may simply reflect the inability to find dateable material in the lowermost flows, especially the more primitive picritic compositions. However, given that these lower volcanic sequences do not exceed 1.5 kilometers in thickness and generally lack weathered flow tops or significant intervals of interflow sediments (Green, 1982; Nicholson et al., 1997) implies that they were likely erupted over a relatively short time period (<1-2 Ma?). However, this may hold true only for the exposed volcanincs, which occur on the flanks of the MCR basin. The increased thickness of reversed polarity volcanics geophysically modeled in the axis of the MCR (up to 10km in the eastern basin, Mariano and Hinze, 1994a), leaves open the possibility that 1115-1110 Ma-aged volcanics may be preserved in the keel of the rift basin.

Tectonomagmatic models developed before the discovery of intrusive ages older than 1110 Ma (e.g., Cannon, 1992; Miller and Vervoort, 1996; Nicholson et al., 1997; Vervoort et al., 2007) reasoned that plume impact occurred around 1110-1109 Ma, the earliest age of most reversed polarity volcanics and many intrusions (Fig. 5), and immediately produced broad rapid volcanism. The older age dates for Nipigon Embayment intrusions (Heaman et al., 2007), which have undepleted mantle signatures indicative of deep (garnet-bearing) sources (Hollings et al., 2007a and 2007c), imply that plume impact may have occurred closer to 1115 Ma. The apparent lack of similar-aged volcanics, which would be expected to have been fed from these hypabyssal intrusions, may indicate that the flows were being eroded during a period of crustal doming. Campbell (2001) argues that the broad (1000-2000 km diameter) thermal anomaly introduced by the arrival of a starting plume head beneath continental lithosphere should lead to widespread uplift prior to the onset of rapid volcanism.

Stage II (1110-1105 Ma) Rapid Plateau Volcanism and Onset of Crustal Underplating - The onset of volcanism and associated intrusions into the MCR represented by the Early Magmatic Stage was characterized by the rapid eruption of intially primitive magmas that gave way to evolved and contaminated compositions. Seismic models imply that volcanic accumulations during this period occurred over a broad area and were not confined to a narrow graben (Cannon, 1992). The compositional

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and chronologic correlations of early volcanic sequences underlain by fluvial to lacustrine sediments in the western part of the MCR suggest that a broad depression had begun to form within the broader crustal uplift. Given the greater thickness of reversed polarity volcanics in the eastern Lake Superior basin, it seems likely that subsidence was initiated earlier or occurred more rapidly than in the west.

This stage likely represents the progressive imbedding of the mantle plume head into the subcontintental lithospheric mantle (SCLM). Magmas generated from the plume initially rose quickly through the cool and brittle lithosphere generating the lower primitive volcanics observed at the bases of most volcanic sequences (Fig. 5; Siemens Creek, Lower Osler, Grand Portage lavas of the lower NE sequence of the NSVG; Units 1 and 2 of the Maimanse Volcanics) and early mafic intrusions (e.g., Disraeli, Kitto diabase, Heaman et al., 2007). The rapid heating of the crust by early mantle-derived melts and the onset of their staging in the lower crust likely led to variably evolved mafic magmas with crustal contamination signatures, the onset of felsic magmatism by crustal anatexis, and the occurrence of plagioclase porphyritic magmas. This produced the compositionally variable and contaminated Hovland lavas of the lower NE sequence of the NSVG, the Central Suite of the Osler, the Lower Kallander Creek lavas of the Powder Mill Group, and Groups 3-5 of the Maimainse Point Formation (Fig. 5). In the intrusive rocks, the latter part of this early stage is represented by the ferrogabbroic Logan sills, the Nipigon sills, the Early Gabbroic Series and Felsic Series of the Duluth Complex, and the alkaline rocks of the Coldwell Complex. At the waning of the early magmatic stages, mafic magmas show the greatest signs of crustal contamination - negative εNd values and elevated Th/Yb ratios (Fig. 6). This is taken as evidence of the onset of extensive magma underplating of the crust resulting in crustal assimilation and anatectic melting to produce felsic magmas. The appearance of plagioclase phenocryst-rich volcanics in the upper parts of the reversed polarity lava sequences and diabase sills (Logan and Nipigon) and dikes is also consistent with deep crustal staging, where high pressures will promote plagioclase flotation (Kushiro, 1980).

Stage III (1105-1101 Ma) Volcanic Hiatus and Extensive Crustal Underplating - During this stage, magmatic activity within the rift was largely dormant, except for periodic rhyolitic volcanism. Deposition of high energy sediments indicates that some degree of vertical subsidence was occurring and that graben bounding faults were beginning to develop. Miller and Vervoort (1996) called this the latent magmatic stage of the MCR because they interpreted it to represent a period of continued mantle plume melting and storage of those melts almost exclusively in the lower crust by extensive crustal underplating. Gravity models over Lake Superior (e.g., Fig. 4, Behrendt et al., 1990, Trehu et al., 1991; Hinze et al,

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Figure 8. Six-stage tectono-magmatic model for evolution of the Midcontinent Rift.interpreted from data presented in Figs. 5 & 6 and discussed in text.

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1992 & 1997; Mariano and Hinze, 1994b; Thomas and Teskey, 1994) indicate that a mafic lens up to 15 kilometers thick occurs at the base of the crust beneath the axis of the MCR. Magmatic underplating, which likely started during the later part of the early magmatic stage, was probably instigated by the heating and anatexis of the lower crust caused by the passage of the earliest mantle-derived magmas coupled with heating from the rising plume. The creation of felsic melts and an increasingly ductile lower crust would have created density and rheologic barriers to impede the passage of mafic melts and promote their ponding at the Moho. Once initiated, mafic magma chambers would have continued to expand as additional rising mantle melts became trapped and triggered more widespread melting of the lower crust. At the peak of the latent/hiatus stage, the lower crust may have been largely impermeable to mafic magmas. Such a process of magma underplating of the lower crust bringing about a volcanic hiatus is envisioned to be a common phenomenon in the evolution of continental flood basalt provinces (Huppert and Sparks, 1988; Cox, 1993; Campbell, 2001).

To the extent that lithospheric thinning and extension was driven by the bouyancy and thermal energy of the starting mantle plume (e.g., Campbell and Griffiths, 1990), perhaps magma underplating actually caused diminished extension of the upper crust by delaminating and structurally decoupling it from the lower crust and lithospheric mantle. Although the resumption of volcanic activity at about 1102-1100 Ma (earlier near the axis of the rift) may have been externally triggered by increased extension of the crust, it seems also possible that “density cleansing of the lower crust” (Huppert and Sparks, 1988) caused by the migration of low density anatectic melts and perhaps thinning and shouldering aside of the ductile lower crust concomitant with magma underplating may have played important roles in allowing basaltic magmas to ultimately emerge from deep crustal magma chambers and bringing on the main magmatic stage.

Stage IV (1101-1094 Ma) Graben-bounded Volcanism and Evacuation of Lower Crustal Magma Chambers -The renewal of volcanic and intrusive activity during the main magmatic stage was characterized by rapid to moderate rates of eruption of uncontaminated (save rhyolite), but diverse magma compositions (Fig. 6). This stage is thought to represent the onset of upper crustal separation, the evacuation of evolved lower crustal magma chambers, and continued, but waning mantle plume melting. The main stage is represented by the nearly 10 km thickness of normally polarized North Shore Volcanic Group lavas in Minnesota and at least 5 km of Portage Lake Volcanics in Michigan, as well as major parts of the Duluth, Beaver Bay and Mellen intrusive complexes. Seismic reflection data clearly show that main stage volcanism was largely confined to rapidly subsiding asymmetric grabens bounded by listric normal faults and transected by accommodation zones (Cannon, 1992; Dickas and Mudrey, 1997).

The first few million years of main stage magmatism involved a differentiated range of tholeiitic basalt and minor felsic compositions (Fig. 6). Many of the earliest magmas were plagioclase-phyric magmas that created the extensive anorthositic series of the Duluth Complex (Fig. 7). Miller and Weiblen (1990) interpreted these plagioclase crystal mushes as derived from lower crustal magma chambers in which plagioclase was bouyant. They further speculated that the nearly pure anorthosite inclusions found in the Beaver Bay Complex are xenoliths derived from the roof zones of these deep chambers.

Although the main stage magmas were initially variably differentiated, they showed little in the way of the crustal contamination that characterized the end of the early magmatic stage (Fig. 6). This probably resulted from the development of thick marginal zones around long-lived lower crustal magma chambers, which effectively insulated the fractionally crystallizing magmas from interacting with the crust. As main stage magmatism progressed, however, magma compositions came to be dominated by primitive to mildly evolved, high-Al olivine tholeiitic basalts, as characterized by the Portage Lake

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Volcanics (Paces and Bell, 1989) and the Schroeder-Lutsen basalts which lie atop the North Shore Volcanic Group (Green, 1972). This suggests that the residence time of mantle derived magmas decreased as the plumbing system of the upper crust became better developed.

Felsic magmas continued to be generated by crustal melting during main stage magmatism. However, in Minnesota,Vervoort and Green (1997) note that the εNd values of rhyolites are progressively more negative up section (Fig. 6). Vervoort et al. (2007) speculate that the source of felsic magmas migrated to shallower crustal levels over time with early melts derived from Paleoproterozoic-aged lower crust and younger melts generated from Archean crustal rocks. In the interior of the deepening grabens, small–volume rhyolites on Keweenaw Peninsula and Michipicoten Island were derived from remelting of early MCR basalts and do not show interaction with older basement crust (Nicholson and Shirey, 1990).

Stage V (1094-1080 Ma) – Thermal Collapse and Sedimentation - As volcanism waned due to the thermal decline of the mantle plume head and the likely detachment of the plume tail due to plate motion, the subsidence of central grabens continued (Fig. 7D). The central axis of the rift grabens became filled with a thick accumulation of alluvial fan, fluvial and some lacustrine sediments to form the Oronto Group. Intermittent volcanism became more localized and more differentiated with the last recorded activity occurring around 1086 Ma. The final melts generated in the Lake Superior region appear to be mixtures of plume and depleted asthenospheric mantle based on the occurrence of N-MORB-like compositions with slightly positive εNd values (Fig. 6). Nicholson et al. (1997) interpreted these magmas as indicating the displacement of the lithospheric mantle and its replacement by vestiges of the plume head and shallow asthenospheric mantle that infilled behind the dissipated head of the plume. Given paleomagnetic data that indicate rapid drift of the Laurentian plate (Swanson-Hysell et al., this volume), it is likely that the plume tail became detached from the plume head soon after the head became imbedded into the SCLM (i.e., during Stage II). Approximately 1.1 Ga mafic dikes are well known in the southwest US (Donadini et al., 2011) which is consistent with paleomagnetic data indicating a general drift of the plate to the northeast relative to a fixed plume. With the thermal decline of the mantle plume head, the rift experienced thermal collapse and sediment loading and the graben became filled with as much as 8 kilometers of sediments.

Stage VI (1080-1040) – Compression and Tectonic Inversion - Cannon (1994) has hypothesized that the tectonic inversion of the rift by reverse faulting on originally normal graben-bounding faults (Fig. 3) probably began around the time of the last volcanic eruptions at about 1086 Ma and may have continued until 1040 Ma. Bornhorst et al. (1988) report Rb-Sr isochron ages between 1060 and 1045 Ma for late-stage mineralization occurring along the Keweenaw Fault (Bornhorst et al., 1988). Cannon (1994) further reasoned that the compression may have been due to rejuvenation of compression within the Grenville Orogen with onset of the Ottowan phase at about 1090 Ma (McEachern and van Breemen, 1993). The northeast-directed regional compression resulted in about 30 kilometers of shortening and the creation of a central horst along most of the southwest arm of the MCR. Modeling of seismic data from the eastern Lake Superior basin and bedrock exposures along the northeast shore indicate that the effects of this compression on the southeast arm of the MCR evidently produced strike-slip motion along the graben-bounding faults, roll-over structures, northeast-trending folds and northwest-directed thrust faults (Mason and Halls, 1994; Samson and West, 1994; Mariano and Hinze, 1994a & b).

Despite the antiquity of the MCR compared to other large igneous provinces, it appears that the mantle plume dynamics and compositions that influenced Mesozoic to Tertiary continental rifting and flood basalt volcanism were fundamentally the same as those that affected the Midcontinent Rift. One

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notable difference between the MCR and more recent plume-influenced LIPs is the prolonged period of magmatism. Shirey et al. (1994) pointed out that the magmatic progression seen in the MCR is similar in many ways to other continental flood basalt provinces. Because the MCR presents one of the most completely exposed sections of flood basalts, by virtue of its incomplete rifting and later tectonic inversion, its tectono-magmatic evolution may provide a test and constraints of general models of plume-generated continental rifting and large igneous province magmatism.

Mineral Deposits Related to the Midcontinent Rift The MCR hosts two major classes of mineral deposits, hydrothermal and magmatic (Nicholson et al., 1992). All important mineral production in this region to date has come from the world-class hydrothermal deposits, whereas mineral deposits related to intrusive igneous rocks have provided only a small fraction of the total mineral production from the rift. That picture is about to change dramatically. At the time of this writing, over a dozen magmatic sulfide deposits are in the advanced exploration, environmental and mine permitting, or mine development stage of activity. The following summary of rift-related mineralization is drawn in large part from Nicholson et al. (1992) and Miller et al. (1995), Smyk and Franklin (2007), and the references therein.

Hydrothermal Deposits

MCR-related hydrothermal deposits include four main types: 1) native copper and silver deposits in basalts and interflow sediments; 2) stratabound copper sulfide and native copper; 3) copper sulfide veins and lodes hosted by rift-related volcanic rocks; and 4) polymetallic veins in the surrounding Archean and Paleoproterozoic country rocks (Fig. 9; Nicholson et al., 1992). The scarcity of sulfur within the rift rocks resulted in the formation of very large deposits of native metals. Where hydrothermal sulfides occur (ie., shale-hosted copper sulfides), the source of sulfur was evidently local sedimentary rocks.

Native Copper and Silver Deposits- The native copper (and minor silver) deposits of the Keweenaw Peninsula have a long history of prehistoric mining and modern production that began in the 1840s. These deposits were the principal source of copper for the United States for many years, yielding more than 5 million metric tons of refined Cu from 1845 until the 1960's when the last mines closed. In addition to the Keweenaw Peninsula deposits, native copper and silver is found in virtually all exposed mafic volcanic sequences associated with the MCR, as well as in Oronto Group and Sibley Group sedimentary rocks (Nicholson et al., 1992; Smyk and Franklin, 2007). Native copper, locally accompanied by small amounts of native silver, occurs in the brecciated and vesicular tops of basalt flows (amygdular lodes), in interflow sedimentary rocks (conglomerate lodes), and in cross-cutting veins (fissures) (White, 1968). The copper typically fills open-spaces or replaces basalt. The major native copper deposits occur in zones where prehnite and pumpellyite are the major alteration minerals and epidote is absent or scarce (see Stoiber and Davidson, 1959; Jolly, 1974; and Livnat, 1983, for discussions of alteration patterns).

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Figure 9. Occurrence of hydrothermal ore deposits associated with the Midcontinent Rift in the Lake

Superior region (modified from Nicholson et al., 1992).

Stratabound Copper Sulfide Deposits - At the White Pine mine in Michigan, which was in production from 1953 to 1996, sedimentary copper sulfide and native copper deposits in the lowermost part of the Nonesuch Formation comprise one of the world's largest shale-hosted copper deposits (Ensign and others, 1968; Brown, 1971; Kelly and Nishioka, 1985; Mauk and others, 1989a, 1989b; Seasor and Brown, 1989; Swenson et al., 2004). In the area of active mining, the ore body is estimated to contain about 200 million tons (181 million mt) of ore with an average grade of 1.1% Cu and 0.25 oz (9 g) Ag/ton (Mauk and others, 1989a). A total of 4.5 billion lbs Cu and 50 million oz of Ag were produced over the life of the mine. At White Pine, the copper is mostly stratabound within the most organic-rich beds of the Nonesuch Formation. Two stages of mineralization are recognized (Mauk and others, 1989a, 1989b): 1) chalcocite layers formed during the main stage of mineralization, probably as a result of late diagenesis, accounting for 80-90% of the contained copper; and 2) during the second stage, native copper was deposited largely in structurally disturbed zones, mostly the result of (post-rift) thrust faulting.

The recently discovered Copperwood deposit is a stratabound copper sulfide (chalcocite) deposit that occurs about 30 km southwest of the White Pine deposit and is also hosted by the Nonesuch Shale. The deposit is on average 2.5m thick with measured, indicated and inferred resources at 33 million metric tonnes averaging 1.60 % Cu and 4.1 ppm Ag for contained metal of 1.165 billion lbs of Cu and 4.32 million oz of Ag (Bornhorst and Williams, 2013).

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Fissure-hosted Copper Sulfide Vein Deposits - Copper sulfide vein deposits occur principally in two areas: 1) near the tip of Keweenaw Peninsula in Michigan, and 2) along Mamainse Point, Ontario (Nicholson et al., 1992). These copper sulfide occur as veins crosscutting the volcanic rocks as well as as stratabound deposits in amygdular flowtops. Chalcocite is the most abundant sulfide, followed by bornite and chalcopyrite. Veins on Keweenaw Peninsula are associated with weak alteration as compared to veins on Mamainse Point which show intense silicification and argillic and propylitic alteration (Nicholson et al., 1992). Vein deposits on Keweenaw Peninsula have been estimated to contain about 6.4 million metric tons of ore with an average grade of 2.3% Cu (Anonymous, 1989). On Mamainse Point, veins mined from the Coppercorp mine yielded about 1.1 million metric tons of ore averaging 1.46% Cu (Nicholson et al., 1992).

Polymetallic Vein Deposits - Silver-dominated, polymetallic quartz-carbonate veins are prevalent in the Thunder Bay area where historically almost 5 million dollars worth of silver ore has been produced since 1846 (Mudrey and Morey, 1972; Franklin et al., 1986; Kissin, 1992; Smyk and Franklin, 2007). The veins typically occur in the vicinity of contacts between Paleoproterozoic sedimentary units (usually Rove Formation shale) and Logan diabase sills. In addition to native silver and acanthite, the veins include a nickel-cobalt sulpharsenide suite of minerals as well as base metal sulfides and gangue minerals of quartz, carbonate, fluorite, and barite. The regional focus of the veins occur in or near crustal-scale listric faults associated with MCR extension that were reversed during late stage compression (Fig 8f; Smyk and Franklin, 2007).

Another type of polymetallic hydrothermal mineralization occurring in the Thunder Bay area is lead-zinc-barite veins that may also include amethystine quartz and uraninite (Franklin and Mitchell, 1977; Smyk and Franklin, 2007). These veins typically occur in the vicinity of the unconformity between the Sibley Group sedimentary rocks and the Archean basement. Uraninite alone in quartz veins and vein breccias with hematite and pyrite are also commonly found in the Archean basement near unconformities (Smyk and Franklin, 2007).

The rift-related hydrothermal deposits can be related to a regional model that includes heating of basinal brines, leaching of metals, and movement of fluids upsection (Nicholson et al., 1992; Swenson et al., 2004; Smyk and Franklin, 2007; Brown, 2006, 2008). With more than 2 million km3 of mafic magma erupted in the rift and a comparable volume of mafic intrusions inferred beneath the rift, a ready and structurally confined supply of mafic source rocks were available for leaching of metals by basinal brines. These brines were heated by a steep geothermal gradient that resulted from the melting and underplating of magma derived from the mantle plume. Hydrothermal deposits were emplaced at least 30-40 m.y. after rift magmatism and extension ceased. This time lag may reflect either the time required to heat deeply buried rocks and fluid within the rift, or may be due to timing of post-rift compression that may have provided the driving mechanism for explusion of hydrothermal fluids from deep portions of the rift. Recently, Brown (2008) suggests that the focusing of ascending copper-rich brines into a narrow segment of Portage Lake strata along a 45-km-long stretch of the Keweenaw Peninsula may be attributed to upward thrusting on the Keweenaw fault during closure of the MCR, forming anomalously hot, steeply dipping aquifers on the Keweenaw promontory, and to the development of a single, dominant thermal plume within the incipiently buoyant brine at the scale of the mine district.

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Magmatic Deposits

The main types of magmatic ore deposits associated with the Midcontinent Rift include: 1) low-grade Cu-Ni-PGE sulfide deposit hosted by gabbroic to troctolitic rocks occurring in the contact zones of Duluth Complex, Coldwell Complex, and other smaller gabbroic intrusions; 2) stratiform PGE “reef” intervals in layered mafic intrusions, 3) high-grade Ni-Cu-PGE sulfide deposits hosted by ultramafic rocks in small ultramafic to mafic intrusions (“conduit-type” magmatic sulfide deposits of Ripley and Lee, 2012); 4) Ti-Fe(-V) oxide-rich ultramafic intrusions in the Duluth Complex; 5) U-REE in small carbonatites cutting Archean rocks; and 6) Cu (Mo)-bearing breccia pipes resulting from local hydrothermal activity around small felsic intrusions (Fig. 10). The ages of the magmatic deposits span the entire range of magmatic activity in the rift from 1115-1086 Ma.

Figure 10. Occurrence of magmatic ore deposits associated with the Midcontinent Rift in the Lake

Superior region (modified from Nicholson et al., 1992).

Cu-Ni-PGE Sulfide Deposits at Contacts of Mafic Intrusions - The copper-nickel sulfide mineralization of the basal portion of the Duluth Complex in Minnesota has been delineated along a 50 kilometer-long belt southeast of Ely, Minnesota (Fig. 7). Exploration of this mineralization, which has been ongoing

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since 1950, has delineated 12 areas of significant grade to be classified as deposits (Hauck et al., 1997; Severson and Miller, 2002). Estimates of the amount of ore (defined as containing 0.1 Cu equivalent) in this belt are as great as 7 billion metric tons with average grades running 0.66% Cu and 0.2% Ni (Listerud and Meineke, 1977; Eckstrand and Hulbert, 2007; Peterson, 2010). The discovery of significant precious metal (Pd+Pt+Au) enrichment in the mid-1980s (Sabelin et al., 1986; Morton and Hauck, 1987), the Duluth Complex ores have come to be recognized as a significant PGE resource as well (Ripley, 1990; Mogessie and Saini-Eidukat, 1992; Ripley and Chryssoulis, 1994; Theriault et al., 1997; Hauck et al., 1997, Severson and Hauck, 2003, Peterson, 2010). Although low in grade, the large tonnage of ore makes the contained metal content of the Duluth Complex deposits world class. Compared to other magmatic ore deposits, Eckstrand and Hulbert (2007) estimated that the Duluth Complex ores rank first in contained copper (~40 billion pounds), comparable to Noril’sk (Fig. 11). They rank third behind Sudbury and Noril’sk in contained nickel and rank fourth behind Bushveld, Noril’sk, and the Great Dyke in Pd+Pt+Au. A resource estimate in December of 2012 indicates that the Maturi deposit alone hosts about one-third of all Duluth Complex resources (13.7 billion lbs copper, 4.4 billion lbs nickel, and 21.2 million oz. of PGE).

The Cu-Ni mineralization occurring at or near the base of the Duluth Complex is hosted mostly by troctolite intruded between slightly older anorthositic series units and the country rocks composed of Paleoproterozoic sedimentary rocks (shale, greywacke, iron formation) and Archean granitoids. Sulfide mineralization consists of mostly pyrrhotite, chalcopyrite, cubanite and minor pentlandite, generally occurring as disseminated grains among silicate phases in the troctolite, but also as veinlets, inclusions in silicates, intergrowths with hydrous minerals, and as rare massive sulfide segregations (Weiblen and Morey, 1976; Foose and Weiblen, 1986; Hauck et al., 1997; Severson and Miller, 2002). Stable and radiogenic isotopic data imply that the copper-nickel mineralization resulted from the interaction of the metal-bearing intruding magma and sulfur-rich fluids derived from desulfurization and dehydration of country rocks and was accompanied by at least some local partial melting and assimilation (Ripley, 1981, 1986; Lee and Ripley, 1996; Ripley et al., 1998; Ripley et al., 2001; Ripley et al., 2008).

Similar contamination-induced basal mineralization occurs in other subvolcanic intrusions in the MCR. These include the Great Lakes Nickel deposit in the Crystal Lake gabbro near the US-Canadian border (Eckstrand et al., 1989), the Mineral Lake Intrusion associated with the Mellen Complex in northwestern Wisconsin (Bakheit, 1981), and the Marathon deposit in eastern gabbro phase of the Coldwell Complex (Good and Crocket, 1994; Good et al., 2010). The Crystal Lake gabbro was emplaced into the Paleoproterozoic Rove Formation just north of the basal Duluth Complex (Fig. 3). The Great Lakes Nickel deposit has a historic (1974) indicated resources of 45,623,00 tons grading 0.344% Cu, 0.183% Ni, 0.0043 oz/ton Pt, and 0.021 oz/ton (Smyk and Franklin, 2007), though a renewed evaluation of the deposit is expected after the property was acquired by Rio Tinto (CNW news release, Nov., 2011). Advanced exploration and resource evaluation is currently being conducted on the Coldwell Complex’s Marathon deposit, which in 2010 was characterized as having Marathon’s optimized proven and probable reserve of 91.45 million tonnes grading 0.832 g/t Pd, 0.237 g/t Pt, 0.085 g/t Au, 0.247% Cu and 1.44 g/t Ag, and containing 2.44 million ounces of Pd, 696,000 ounces of Pt, 251,000 ounces of Au, 497 million lbs of Cu and 4.23 million ounces of Ag (Marathon PGM Corporation news release, Jan., 2010).

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Figure 11. Tonnage of ore versus grades of nickel, copper and PGE (Pt+Pd+Au) in global occurrences of

magmatic ore deposits including the Duluth Complex. Diagonal lines indicates tonnes (Ni,Cu) or kilograms (PGE) of contained metal. Diagram from Peterson (2010) modified from plots of Eckstrand and Hulbert (2007).

Stratiform PGE “Reef” Deposits - Economic concentrations of platinum group elements (PGEs) in meter-thick, stratiform, sulfide-bearing horizons (PGE reefs) have long been known to be associated with ultramafic-mafic layered intrusions such as the Bushveld and Stillwater complexes (Naldrett, 1993), typically occurring near the transition from ultramafic to mafic rocks. The discoveries of stratiform precious metal mineralization in the Skaergaard intrusion and related bodies of East Greenland (Bird and others, 1991; Aranson and others, 1997; Andersen and others, 1998; Andersen, 2006) demonstrated that PGE reefs may also exist in tholeiitic mafic layered intrusions. Stratiform PGE mineralization in well-differentiated tholeiitic intrusions are similar to classic PGE reef deposits hosted by ultramafic-mafic complexes, such as Bushveld and Stillwater complexes, in that they occur as sulfide-poor (<1 wt.%), PGE-rich intervals that are meters in thickness and are conformable with igneous layering. Stratiform PGE mineralization in tholeiitic intrusions, termed Skaergaard-type PGE mineralization by Prendergast (2000), differs from the classic PGE reefs, however, by being: 1) exclusively associated with mantle plume-influenced, continental rift environments; 2) of Middle Proterozoic age or younger, 3) associated with aluminous, olivine tholeiitic parent magma compositions that experience Fenner-type crystallization differentiation; 4) hosted by ferrogabbroic cumulate rocks, 5) associated with Cu-rich, Ni-poor sulfide, and 6) associated with significant Au that is stratigraphically offset above peak PGE concentrations (Miller and Severson,2002b). Whereas there is considerable disagreement about how classic PGE reefs

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formed (Cawthorn, 1998), most believe that Skaergaard-type reefs are orthomagmatic, i.e., they formed by the saturation, exsolution and settling of sulfide melt from silicate magma. The presence of many well differentited layered intrusions of tholeiitic composition associated with the 1.1 Ga Midcontinent suggests that it is fertile ground for exploration for this type of mineralization.

Recognizing the remarkable similarities in the closed-system crystallization histories of the Skaergaard intrusion and the Sonju Lake intrusion (SLI) of the Beaver Bay Complex in northeastern Minnesota, Miller (1999) conducted a chemo-stratigraphic study of the SLI to determine if it may contain a Skaergaard-type reef. Elevated Pd and Pt concentrations (50-400 ppb) were discovered within an 80 meter thick interval hosted by oxide gabbro cumulates and situated stratigraphically below an abrupt increase in Cu (100 - 600 ppm). In 2002, Franconia Minerals drilled three cores through the PGE reef and supported a petrographic, lithochemical, and mineral chemical study by Greg Joslin for his MS thesis. Joslin (2004) showed that anomalous concentrations of Pd and Pt occurred over an 85-meter-thick interval and that peak concentrations were cyclical, offset from each other and from the abrupt increase in Cu, and correlative among the three cores (Fig. 12). The detailed chemostratigraphy also showed that gold was concentrated at the abrupt increase in Cu (Cu-Au break). Joslin concluded Cu and precious metal mineralization was generated by sulfide saturation that was passively triggered by closed-system fractional crystallization of the SLI parent magma. He further concluded that the Pt and Pd concentrations reflected primary mineralization, whereas the Cu and Au break represented a sulfide dissolution front created by the migration of deuteric fluids through the mineralized zone. The broad thickness of PGE mineralization is thought to result from the slow settling of small amounts (~0.1 wt.%) of sulfide liquid that would be generated from a passively saturated tholeiitic magma.

Figure 12. Stratigraphic variations in concentrations of Cu, Au, Pt, and Pd across the Precious Metal

Zone of the Sonju Lake Intrusion observed in three Franconia drill cores. The core are correlated at the Cu-Au break. However,Miller (2011) argues that complex Pt and Pd peaks are primary orthomagmatic signatures of mineralization and that the Cu-Au peak is a secondary feature representing a sulfide dissolution front. Cu values in ppm, Au, Pt, and Pd values in ppb. From Joslin (2004).

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Another MCR layered intrusion that shows evidence of a stratiform PGE reef is the Layered Series

at Duluth (DLS; Miller, 2011). This 4-km-thick sheet-like body, which is the type layered intrusion of the Duluth Complex, is well differentiated, but displays cyclical phase and cryptic layering that indicates that it was open to multiple episodes of magma venting and recharge. Chemostratigraphic variations in chalcophile elements through the DLS indicates that sulfide saturation likely occurred in the medial section of the body where troctolitic cumulates begin to cyclically grade into gabbroic cumulates, a section termed the Cyclic Zone. Given the lack of cyclical cryptic layering attending the phase layering of the Cyclic Zone, Miller (2011) interpreted this phase layering as largely caused by decompression attending magma venting under low lithostatic pressures. He further speculated that under water saturated conditions, decompression may have also triggered sulfide saturation and PGE reef mineraliation in the lower part of the cyclic zone. If such mineralization is found, its formation by more dynamic processes may result in a more economic reef by virtue of its being thinner and higher grade.

Stratiform PGE mineralization in mid- to upper levels of mafic layered intrusions have been reported in other MCR-related layered intrusions (Seagull – Heggie, 2005; Partridge River - Geerts, 1991, 1994; South Kawishiwi – Severson, 1994; BIC – Foley, 2011; Tamarack – Goldner, 2011; Echo Lake – Billard, 2003). Of these, only the BIC occurrence, which is hosted in oxide gabbro, appears to be a Skaergaard-type PGE reef formed by sulfide saturation passively triggered by fractional crystallization. Most of the other reef occurrences are what Miller and Severson (2002b) termed stratabound PGE mineralization because they are commonly associated with a particular rock unit, usually olivine cumulates that seem to mark a recharge event into the magma chamber (Severson, 1994). Other ways that stratabound reefs differ from “Skaergaard-type reefs” is that sulfides are not as depleted in Ni and sulfide typically becomes undersaturated in the overlying unit. With most uncontaminated primitive tholeiitic magmas being initially undersaturated in sulfide, magma recharge in and of itself should not be able to cause sulfide saturation. The fact that most of these ultramafic-hosted PGE reef occurrences are associated with intrusions that have significant basal contact sulfide mineralization, it seems likely that stratabound reef mineralization is due to sulfide contamination as the recharging magmas pass through the basal mineralized zones or sulfide-enriched conduits. That sulfide eventually becomes undersaturated may indicate that only the leading edge of the recharging magma becomes oversaturated.

Ni-Cu-PGE Sulfide Deposits in Small Ultramafic Intrusions – Clearly one of the most stimulating mineral discoveries in the past 15 years has been the recognition of high-grade Ni-Cu-PGE mineralization associated with small ultramafic-mafic intrusions (“conduit-type” magmatic sulfide deposits; Ripley and Lee, 2011) emplaced during the initiation and early stages of MCR magmatism. The currently identified intrusions include Kitto, Hele, Seagull, Disraeli and Current Lake intrusions and the Jackfish Island, Shilabeer, and Riverdale sills in the Lake Nipigon-Thunder Bay, Ontario area; the Eagle, BIC, and Roland Lake intrusions in upper Michigan; and the Tamarack intrusion of east central Minnesota (Fig. 3). Although several of these ultramafic intrusions were identified decades ago (e.g, Eagle was known as the Yellow Dog peridotite, Klasner et al., 1979; Kitto and Disraeli were identified as picritic intrusions by Sutcliffe, 1987; the unexposed Tamarack intrusion was known from one shallow drill core acquired by the Minnesota Geological Survey, Southwick et al., 1986), the presence of significant mineralization in the basal contact zones of many of these intrusions was not discovered until about 15 years ago when exploration drilling was conducted. Summarized here are brief descriptions of the five best studied intrusions to date – Seagull, Current Lake, Eagle, BIC, and Tamarack.

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The first discovery was made of the Seagull intrusion by exploration drilling of aeromagnetic anomalies southwest of Lake Nipigon in 1997 (Smyk and Franklin, 2007). A detailed petrologic study of several drill cores by Heggie (2005) shows the lopolithic intrusion to be composed of a 800 m-thick keel comprising olivine cumulates (dunite, peridotite,lherzolite) overlain by a 100 m-thick sheet of feldspathic pyroxenite to gabbro. Middleton and Heggie (2005) reported a basal contact zone of disseminated sulfide with grades up to 0.35% Cu, 0.25% Ni, and 3.6 ppm Pt+Pd (~1:1 ratio). Several PGE-rich stratiform horizons of finely disseminated sulfide in 5 meter-thick intervals occur between 100-200 meters above the basal contact and run as high as 0.68% Cu , 0.36% Ni, 5.5ppm PGE. Heggie (2005) interpreted these upper stratiform horizons to have formed by recharge into the chamber by S-contaminated magmas.

The Current Lake intrusion north of Thunder Bay, Ontrario was discovered in 2006 by Magma Metals Ltd. when angular peridotite boulders enriched with Ni-Cu-PGE sulfide were traced up-ice to a prominent linear aeromagnetic anomaly. An extensive drilling program over the past six years has recovered over 145,000m of core which in 2009 defined an indicated resource of 4.6 Mt grading 1.35 g/t Pt, 1.27 g/t Pd, 0.32% Cu and 0.22% Ni (MacTavish and Smyk, 2010). A May 2012 press release reports an indicated resource of 10.3 Mt with a platinum equivalent grade of 2.4 g/t. The extensive drilling shows the mineralized intrusion to be a sub-horizontal, 30-50 m wide by 30-70 m thick chonolith that gradually deepens and widens to the south (Goodgame et al., 2010). Disseminated mineralization occurs throughout the peridotitic intrusion, which is dominantly composed of olivine cumulates with interstitial augite and plagioclase. An inclusion-rich quartz leucogabbro commonly occurs at the margins of the mineralized periodote and is interpreted to represent a precursor intrusion that has been strongly contaminated and hydrothermally altered (Chaffee et al., 2012).

By far the richest Ni-Cu-PGE mineralization discovered among the MCR ultramafic-mafic intrusions is associated with the Eagle Deposit of Upper Michigan. Discovered in 2002 by Kennecott Exploration, massive to semi-massive sulfide hosted by peridotitic to olivine melagabbroic rocks occuring in a narrow funnel-shaped intrusion within Paleoproterozoic black shales (Ware et al., 2009; Ding et al., 2010). A U-Pb age of 1107.2 ± 5.7 Ma for badellyites has been reported by Ding et al. (2010). Its elliptical areal extent of the intrusion is about 480 m long by 100-200 m wide. Eagle East is a similarly shaped, though less richly mineralized intrusion that occurs 0.6 kilometers east of the Eagle body. A 2009 Kennecott report gave a resource estimate for Eagle of 4.05 million tons with an average grade of 3.57% Ni, 2.9% Cu, 0.10% Co, 0.28 ppm Au, 0.73 ppm Pt, and 0.47 ppm Pd (Ding et al., 2010). Geochemical and mineral chemical studies by Ding et al. (2010) concluded that the intrusion formed in a dynamic conduit setting from at least three magma pulses which were olivine-phyric and sulfide-oversaturated due to country rock contamination.

Occurring in a similar geologic setting as Eagle, but 45 kilometers to the east (Fig. 3), is the small broadly funnel-shaped BIC (Bovine igneous complex) intrusion (Rossell, 2008; Foley, 2011; Foley et al., this issue). Unlike other MCR ultramafic intrusions, the BIC intrusion is relatively well exposed in 1 x 0.5 km hillock, though mineralization is not evident at surface. Mineralization near the basal contact of the intrusion was discovered in drill core by Kennecott in 1995 though this and subsequent drilling has yet to reveal economic mineralization. Rossell (2008) reported one 16.47m intersection averaging 0.88%Cu, 1.00%Ni, 0.679ppm Pt, 0.991ppm Pd and 0.104ppm Au. A recent mapping and petrologic study (Foley, 2010; Foley et al., this issue) shows the intrusion to composes of two differentiated cycles of cumulates: 1) Ol Cpx+Ol , 2) Ol Cpx+OlCpx+Ox±Ol Pl+Cpx+OxPl+Cpx+Ox+Ap. Foley (2010) interpreted these cycles to indicate to main pulses of a high-Mg tholeiitic magma that underwent fractional crystallization. Each pulse produced basal contact sulfide mineralization due to country rock

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contamination attending initial emplacement and the second cycle also produced sulfide-poor, stratiform (reef-style) PGE-enriched mineralization where oxide appears as a cumulus phase.

The Tamarack intrusion is an unexposed mineralized ultramafic intrusion located about 50 miles west of Duluth, Minnesota. After shallow drilling by the Minnesota Geological Survey discovered unmineralized ultramafic rock generating a linear aeromagnetic anomaly in the Paleoproterozoic Animikie Basin (Southwick et al., 1996), Kennecott Exploration began exploration drilling of the anomaly in 2001 and soon encountered significant Ni-Cu-PGE mineralization. Like Eagle and BIC, the Tamarack intrusion was emplaced into pyritic Paleoproterozoic black slates during the early magmatic stage of the MCR (U-Pb baddeleyite age of 1105.6 ± 1.3 Ma; Goldner, 2011). Taranovic et al. report δ34S values between 0.8 and 2.8 indicating up to 50% contamination by external sulfur. Drilling and geophysical data indicate that the Tamarack intrusion has a tadpole-like shape that is about 13 km long and is between 1 and 4 km. The narrow tail area of the intrusion, which is the site of greatest exploration drilling, is composed of exclusively of lherzolitic rock types (cumulus Ol with interstitial Cpx, Opx, and Pl), which Goldner (2011) interprets to have formed in two pulses. The wider “body” area at the southeastern end of the intrusion is composed of a wider variety of rock types ranging from lherzolite to granophyric gabbronorite that Goldner (2011) concludes formed by closed system fractional crystallization. In a March 2009 press release, an estimated reserve of 9-11mt was reported with a grade range of 1.0-1.1 % Ni and 0.6-0.7% Cu, but a full resource estimate has not been released.

Ti-Fe(-V) Oxide Deposits - High concentrations of vanadium-rich ilmenite and titanomagnetite minerals occur in many gabbroic intrusions related to the MCR. In the Duluth Complex, Hauck et al. (1997) identified three distinct occurrences: 1) banded or layered, oxide-rich, meta-sedimentary inclusions in mafic and ultramafic rocks; 2) banded or layered oxide segregations (cumulates) in mafic rocks; and 3) discordant oxide-bearing ultramafic intrusions (OUIs) with semi-massive to massive oxide zones. Titaniferous iron ores were first discovered in the Duluth Complex around 1867, at about the same time as the initial discovery of the Mesabi Range iron ores (Winchell, 1897). The earliest discoveries took place along the northern margin of the Complex in Cook County, MN. Grout (1949-50) estimated that along the northern margin there are 81.6 million tons of low-grade titaniferous magnetite ore (in 14 bodies) with an average grade of 12-14% TiO2 (Hauck and others, 1997b). Oxide mineralization is also associated with late plug-like oxide ultramafic intrusions (OUI) along the western margin of the Duluth Complex (Severson and Hauck, 1990; Severson, 1995). The OUIs were initially discovered by drilling magnetic highs during Cu-Ni exploration. Listerud and Meineke (1977) estimate that 220 million tons of oxide material, with >10% TiO2, are present in at least three of the OUI bodies. However, an additional nine areas are known to contain titaniferous iron ores and are not included in their original estimate (Severson, 1995). A recently released resource estimate for two OUI bodies currently being explored give indicated resources of 58.1Mt @ 16.6% TiO2, 18.8% Fe2O3 and inferred resources of 65.3Mt @ 16.4% TiO2, 19.4% Fe2O3 for the Longnose deposit and inferred resources of 45.1Mt @ 15.0% TiO2, 14.74% Fe2O3 for the TiTac deposit (formerly the Section 23 occurrence; Caredero Resources Inc, news release, Jan, 2012).

Titaniferous oxide concentrations have also been recognized in MRS gabbroic intrusions in northern Wisconsin (Nicholson et al., 1992). The Round Lake intrusion, although not exposed, is inferred from geophysical data and limited drilling to be at least 8 km long and less than 1.5 km wide (Stuhr and Cameron, 1976 ). Drill core shows the intrusion to be layered and consist of magnetite-troctolite, magnetite, anorthositic olivine gabbro, and mafic pegmatite. Titaniferous magnetite is present throughout the intrusion but is concentrated in an oxide-rich core.

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U-REE Mineralization - Intrusions of carbonatite and alkalic complexes that host U-REE mineralization are presently known only north of Lake Superior along the Kapuskasing structural zone and along a north-trending zone north of the Coldwell Complex (Fig. 3; Weiblen, 1982; Sage, 1987, 1988; Nicholson et al., 1992; Smyk and Franklin, 2007). MCR intrusions associated with U-REE mineralization include the alkaline phases of the Coldwell Complex (1108 Ma; Heaman and Machado, 1992), the Killala Lake alkali complex (Sage, 1988), the Prairie Lake carbonatite complex (~1030 Ma; Bell and Blenkinsop, 1987) and the Firesand River alkalic complex (1060 Ma; Bell and Blenkinsop, 1987). A crustal lineament defines a northerly trend that connects the carbonatite-alkalic complexes of Coldwell, Killala Lake, and Chipman Lake (Fig. 3). The Prairie Lake carbonatite lies at the intersection of two well-defined lineaments, including one that connects the Prairie Lake to the Killala Lake Complex (Sage, 1987).

Recent interest in rare earth and related elements has led to active exploration in the Prairie Lake carbonatite and the Coldwell Complex, Ontario. The Prairie Lake deposit is among the top 10 carbonatite-hosted niobium deposits in the world, although its resources have not yet been fully defined. Other elements of interest include tantalum (Ta2O5), uranium (U3O8) and rare earth elements (REE) (Nuinsco Resources Limited, http://www.nuinsco.ca/projects/prairie-lake/ as of May 6, 2012). Exploration for rare earth elements in the Coldwell Complex have focused on niobium, zirconium, yttrium, and rare earth elements (Rare Earth Metals Inc., http://www.rareearthmetals.ca/article/coldwell-complex-169.asp as of May 6, 2012)

Cu (Mo)-bearing Breccia Pipes – Disseminated Cu and Cu-Mo mineralization occurs in four breccia pipes in the Tribag area on the eastern side of Lake Superior, near Mamainse Point, Ontario (Fig. 3). These oval breccia pipes occur within Archean metavolcanic basement along a granite-greenstone contact, and within approximately 6 km of the contact of the Archean rocks with overlying Keweenawan basalts and sedimentary rocks (Blecha, 1974; Norman and Sawkins, 1985). Numerous basaltic and felsic dikes of probable Keweenawan age intrude the highly fractured Archean basement and many predate brecciation (Norman and Sawkins, 1985). The breccia pipes have been dated at 1055 _+ 35 Ma based on a K-Ar isotopic age determination for muscovite from altered rocks that contain chalcopyrite, quartz, and carbonate (Roscoe, 1965).

The sulfide deposits were discovered in 1954 with additional exploration and development work carried out in the 1960's. Two breccias pipes were mined from 1967 until 1974, during which time about 35.2 million lbs (16,000 mt) of copper were produced from 6.4 million tons (5.8 million mt) of ore averaging 2.75% Cu (Blecha, 1974; Norman and Sawkins, 1985). Sulfide mineralization in the Tribag deposits consists of chalcopyite and pyrite with minor sphalerite, galena, tetrahedrite, and molybdenite. Blecha (1974) also reported small amounts of recoverable silver and gold. Areas of Future Research

In December, 2010, an informal working group of academic, government, and industry geoscientists met in Duluth to discuss where future research should be directed to improve our understanding of the geology, mineralization and tectonomagmatic evolution of the MCR. Action items for future research put forth during those discussions provide a fitting way to conclude this summary and review.

• Develop and manage a digital GIS compilation of geological, structural, lithologic, lithochemical, mineral chemical, geochronologic, physical property, and mineral deposit data for the entirety of the exposed MCR. Such a compilation has been informally maintained by the US Geological Survey, the Minnesota Geological Survey, and the Ontario Geological Survey for almost two

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decades. A preliminary release of some of this database is expected in the near future and will provide an invaluable foundation for future studies and exploration of the MCR.

• Continue to conduct detailed mapping throughout the MCR where appropriate. Many areas of good bedrock exposure within the MCR still remain to be mapped in detail (1:20,000-50,000). With the recent discovery of mineralized ultramafic intrusions throughout the poorly mapped Quetico subprovince of Ontario, detailed mapping is particularly needed here.

• Acquire (and re-acquire) more high-precision U-Pb dates from MCR volcanic and intrusive rocks to better establish temporal correlations between volcanic sequences and between volcanics and intrusive units; to better constrain rates of emplacement and eruption rates; to verify the accuracy of seemingly anomalous ages; and to establish the areal extent of MCR magmatism. Inter-laboratory comparisons should be run in order to establish confidence in results. Additionally, establishing good field relationships, to the extent possible, is necessary before acquiring samples for radiometric dating. Two groups that will benefit from detailed age dating are: 1) Mamainse Point Formation in Ontario and 2) the Powdermill Group in Michigan and Wisconsin.

• Accumulate more complete and higher resolution chemostratigraphic databases through the various volcanic sequences. An evaluation of the chalcophile element chemostratigraphy of volcanic sequences (tied to accurate chronologic data) has elsewhere shown promise for identifying potential mineralized intrusions at depth (e.g., Lightfoot and Keays, 2005; Keays and Lightfoot, 2006).

• Conduct geochemical modeling of volcanic rocks in order to understand the magmatic sources and processes that generate the compositional diversity of MCR magmas. With geophysical models and geologic observations indicating magmatic staging occurring at various depths in the crust, it would be interesting to understand the extent to which that diversity is generated by high pressure vs. low pressure fractional crystallization. Other petrochemical questions include the origin of high-Ti vs. low-Ti basalts and the significance of variable Nb-Ta anomalies.

• Within mafic layered intrusions of the MCR, acquire high resolution PGE assay data (<1ppb accuracy) in order to assess the potential for PGE reef deposits by monitoring the stratigraphic variation in mineralization indicators such as the Cu/Pd ratio (Barnes and Lightfoot, 2005).

• Acquire new high resolution geophysical surveys and physical properties data (e.g. specific gravity, magnetic susceptibility, polarity) for MCR rocks and country rocks. Particularly useful surveys would include high resolution aeromagnetic surveys (<200m spacing) in areas flanking the MCR (e.g. Animikie Basin, Baraga Basin, Archean basement around Thunder Bay); airborne gravity surveys; depth to bedrock surveys using newly developed passive source seismometers; and high precision paleomagnetic surveys as recently conducted by Hysell-Swanson et al. (2009; this volume). Evaluation of the recent Earthscope wide array passive seismic survey data will likely lead to a better understanding of the deeper structure and composition of the crust, especially the character of the mafic underplated keel inferred by other geophysical models.

• To identify prospective areas under a thick glacial cover, conduct detailed surficial mapping over and down-ice of the MCR and evaluate regional till sample surveys for metallo-magmatic indicator minerals - Cr-spinels, Fe-Ti oxides, sulfides, olivine (analyzed for Fo and Ni%)

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• Characterize the effects of deep weathering on the chemistry and magnetic properties of MCR intrusive rocks. Understanding the latter effect would help to better interpret aeromagnetic data, which is critical to making geological interpretations in areas of significant glacial cover.

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American Midcontinent Rift System: New interpretations for western Lake Superior, northwestern Wisconsin, and eastern Minnesota. In: Ojakangas, R.J., Dickas, A.B., Green, J.C., (eds.) Middle Proterozoic to Cambrian Rifting, Central North America: Geological Society of America Special Paper 312, p.47-72.

Andersen, J.C. Ø., 2006, Postmagmatic sulphur loss in the Skaergaard Intrusion: Implications for the formation of the Platinova Reef. Lithos v. 92, p. 198–221

Andersen, J.C.Ø., Rasmussen, H., Neilsen, T.F.D., and Rønsbo, J.G., 1998, The Triple Group and the Platinova gold and palladium reefs in the Skaergaard Intrusion: stratigraphic and petrographic relations: Economic Geology, v. 93, p. 488–509.

Anderson, R.R., 1997, Keweenawan Supergroup clastic rocks in the Midcontinent Rift of Iowa In: Ojakangas, R.J., Dickas, A.B., Green, J.C., (eds.) Middle Proterozoic to Cambrian Rifting, Central North America: Geological Society of America Special Paper 312, p.211-230.

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Bakheit, A.K., 1981, Petrography of Cu-Ni mineralization in Mineral Lake area, Ashland County, Wisconsin. University of Wisconsin-Madison, M.S. thesis, 104 p.

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Barnes, S.-J., and Lightfoot, P., 2005, Formation of magmatic nickel sulfide ore deposits and processes affecting their copper and platinum-group elementcontents: Economic Geology, 100th Anniversary Volume, p. 179-213.

Behrendt, J.C., Green, A.G., Cannon, W.F., Hutchinson, D.R., Lee, M.W., Milkereit, B., Agena, W.F., and Spencer, C., 1988, Crustal structure of the Midcontinent rift system—results from GLIMPCE deep seismic reflection profiles: Geology 16, p. 81-85

Behrendt, J.C., Hutchinson, D.R., Lee, M.W., Thornber, C.R., Trehu, A.,Cannon, W.F., and Green, A.G., 1990, Seismic reflection (GLIMPCE) evidence of deep crustal and upper mantle intrusions and magmatic underplating associated with the Midcontinent Rift System of North America: Tectonophysics 173, p. 617-626.

Bell, K. and Blenkinsop, J., 1987. Archean depleted mantle: Evidence from Nd and Sr initial isotopic ratios of carbonatites. Geochim. Cosmochim. Acta, 51: 291-298.

Bercovici, D., and Mahoney, J., 1994, Double flood basalts and plume head separation at the 660-kilometer discontinuity: Science 266, p. 1367-1369.

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